They may have considered that by bundling Figma's functionality into their cloud subscription, with the number of new subscribers that would bring, that revenue attributable to that added functionality would be much more than 400 million annually.
It would need to be much, much more than 400 million to justify the purchase, and I'm not sure numbers of that size will ever materialize.
Another way to look at it is, what amount of investment would Adobe need to make to bring equivalent functionality in-house. Certainly less than 20 billion, by an order of magnitude at least. But then you have to wait, and manage with inspiration and dedication, and possibly fail. Not for the faint of heart.
Also this is being reacted too negatively, they are paying a HUGE price for a company that only has a AAR of 400 million a year.