Would it be actually legal for the Figma owners to sell the company to Adobe, create the new company, say Figma2.0 and create similar(better) product from scratch, also advertise it as creators of original Figma come up with Figma2.0?
If anything, they probably want to tackle some other problem.
Not possible for this type of acquisition. It was reported that this was a cash+stock deal. The typical acquisition contract means the Figma founders would likely have "golden handcuffs" to stay on as employees of Adobe where their Adobe stock would vest over several years. (Say 4 years.) The contract also means they have a standard "no compete" clause and can't quit and start similar company for several years.
2. The buyer will generally put terms in the M&A contract that forbids the main guys of the acquired company from building or working on competing products for a year or two