Is this a valid argument against clean meat?

I think this would be an unusual way to think about the issue, and is likely to make very little overall difference to animal welfare.

Consider the population of Americans (I'm using my country as one example, with made-up statistics) who don't care how much meat they consume. Let's say that's 90% of the country.

Of the remaining 10%, what fraction eat less meat for religious reasons that won't be affected by this argument? What fraction eat less meat for health reasons? What fraction eat less meat because they don't like the idea of hurting animals, even if the overall "problem of animals being hurt" may be "solved" by cultured meat (according to their view of the world)?

None of those people should care about this argument. The only people who seem likely to eat more meat as a result of cell-based meat development are those who currently eat very little meat because they don't like the idea of contributing to the abstract "problem of factory farming", rather than caring about the suffering of particular animals, religious prohibitions, their own health, etc.

These people probably exist, but I'd think that they are very, very rare. (Probably overrepresented in EA, but still rare.)

This doesn't really answer your question in a qualitative way but could be useful data(?) anyway.

For me, it goes the opposite way–it's easier to not eat meat right now when I tell myself I'm not giving it up forever, just until there is an ethically acceptable source.

Also, I expect that if an alternative to factory farming becomes commonplace, the next generation's view of factory farming will shift to something like "how could this ever have been accepted", and I kind of don't want to be on the wrong side of that.

These are two pretty idiosyncratic potential cultured meat-based reasons to not eat meat now. But on the other hand I'm someone for whom your interlocutor's argument doesn't have an impact, which might help you decide how prevalent people for whom it does have an impact are.

I am not sure if there has been a study or survey on this subject so far, but it seems like the kind of thing effective altruists would create a study or survey to assess.

People I know who are concerned about factory farming but who also still eat meat and are also aware of clean meat are looking forward to progress in clean meat, and intend to buy it as it becomes able to replace demand for meant from animals. In the meantime, they intend to still eat meat from animals, to varying degrees. I don't think most of them are excusing clean meat coming out soon as an excuse to not worry about climate change, or keep eating meat now, since they were probably going to keep eating meat in the present either way. They have just expressed a willingness to switch to clean meat in part of out significant concern with factory farming when cleat meat becomes as convenient as animal meat.

Even if there are minor negative short term effects (and while there almost certainly are >0 people in the world following the cited logic, I'm sure they're responsible for far less than even 0.1% of global meat consumption), it still seems to me like the most likely solution to factory farming in the long term, and thus the expected benefits of cultured meat very vastly outweigh the cost that is implied by that argument.

1) I believe most ethical vegetarians avoid meat in order to not actively cause any harm to animals, and not so much in order to solve factory farming. And for the former, the advent of cultured meat in the future doesn't make that much of a difference for their present behaviour.

2) People committed enough to actually think about how their actions contribute to creating a more vegetarian (or at least factory farm-less) world, and thus people who would in theory be affected by the given argument, probably aren't the same people that would think "oh well this issue is being dealt with by others already, nothing to do here". Plus 1) still applies here, as people with such a level of commitment almost certainly also want to avoid personally causing harm to animals.

3) the exception to 1) and 2) may be a few effective altruists (or people with similar mindsets) here and there, who get to the conclusion that sticking to a vegetarian/vegan diet is not worth it for them personally given the apparent tractability and non-neglectedness of the problem, but we're probably talking about dozens or at most hundreds of people around the globe at this point, and if they actually exist this would even be a good sign, as the reason these people would make this decision would be the fact that, well, cultured meat solves the issue of factory farming in such an effective manner that their personal contribution via ethical consumption would have a smaller marginal impact than whatever else they decide to do.

Admittedly a lot of speculation on my part, but what it comes down to is that the argument, while probably playing some non-zero role, just hasn't enough weight to it to justify changing one's view on cultured meat.