It's a great question from a P&L standpoint, but like you said: the board of the company represents the shareholders, and it's likely that $20 billion was a number _far_ above what they viewed the company's value - therefore accepting that type of deal would've made 100% sense to maximize return for those investors.
Note that many employees are also shareholders, and therefore would've banked quite a bit of $ if this went through.
I think the better questions are: did Adobe _need_ Figma? (We'll see...) and why did Adobe overpay by so much?
EDIT: after some quick searching, estimates place the value of Figma at $8-9 billion...making the offer of $20B virtually impossible to reject.