This question has been answered to some extent by Aaron Gertler, regarding the 2018 version of Double Up Drive, here.

This year, it seems like the Drive turned out to be counterfactual for all money raised after $2.4 million, but not necessarily before (we don’t actually know).

Elsewhere on this forum, Aaron also said:

[...] finding out whether a match is actually counterfactual can be a really big deal for the community; I wish I'd worked harder to confirm with the Double Up Drive team whether their match was counterfactual (I think the answer turned out to be "yes", in which case I should have done more promotion, but I'm not actually sure).

I also asked an organiser (Dan Smith) about it directly, on Twitter. He said:

I do the best I can to make Double up Drive a true match. I personally only donate my funds to these causes when the drive fills out. I also think for donors that getting to choose which fund gets matched is impactful relative to other ways to donate.