Curious: Do they really need 5 million Euro?
Now, I cannot give you a line-by-line account of the budget estimate that went into that number (you can look at the 2023 report https://blog.joinmastodon.org/2024/12/annual-report-2023/ with the 2024 report coming sometime in Q1 of this year I think, more timely anyway; and you'll see that's a big upswing / optimistic forward-looking goal); but, it is lower than some other non-profits, foundations, and other efforts elsewhere.
So by all means ask whether that number is valid, but also look around at other OSS efforts. I'd also point out that these are critical times for the future of the open social web, and we (all of us) need to sustain it.