To me it seems they understood longtermism just fine and just so happen to disagree with strong longtermism's conclusions. We have limited resources and if you are a longtermist you think some to all of those resources should be spent ensuring the far future goes well. That means not spending those resources on pressing neartermist issues.
If EAs, or in this case the UN, push for more government spending on the future the question everyone should ask is where that spending should come from. If it's from our development aid budgets, that potentially means removing funding for humanitarian projects that benefit the worlds poorest.
This might be the correct call, but I think it's a reasonable thing to disagree with.
I don't think the arguments are fallacious if you look at how strong longtermism is defined:
Positively influencing the future is not just a moral priority but the moral priority of our time.
See general discussion here and in depth discussion here
Perhaps they should have made that distinction since not all EAs take the strong longtermist view - including MacAskill himself who doesn't seem certain.
The article was on MacAskill’s book which doesn’t argue for strong longtermism but longtermism.
However, I think the acknowledgement and critique of strong longtermism is necessary.