But they also only sold a very small number of shares at that valuation, vs all of them at $20B to Adobe.

this is the answer

Right, which gave them 19B (ish) to play with and they are an independent competitor to Adobe.

Mergers trigger layoffs

Figma raised $1.2B in their IPO. Total shares listed != money raised, not by a long shot. Most shares are just to give liquidity to existing shareholders of the company.

If they have to issue shares the higher valuation is significant

The employees will have to wait 180 days (at least that's the standard) before selling any shares, so they usually feel the effects of a "bounce".