It strikes me that the $20bn is less about the value of Figma as a business entity and more about the potential damage that Figma can issue on Adobe.
You can't compete against an essentially "acceptable" product that is free.
Adobe Photoshop is/was a great tool. But it has become a complicated, pain in the butt for novices, aimed at corporate/business users.
Just look at the numbers - Adobe earns $18bn revenue per year and $6bn profit.
Figma earns $400m per year revenue. It's a pure annihilation of the competition situation. Give it two years, some free features will be scaled back, you'll need a new account style. They will relaunch some features...
If only someone else takes the initiative and creates a freemium Figma clone......DOOO IT!