They most likely have dual trigger RSUs, so the second trigger is typically a liquidity event.
You are talking about single trigger RSUs with only time based vesting.
Also a 83b means you pay taxes at the time of issuance rather than vesting. If these folks received their shares post founding, that means that there might be substantial tax burden for them when they received these shares if a 83b was filed. This is why most folks do not opt for a 83b after a substantial FMV has been established for their shares. This is also why double trigger RSUs are popular, so you can actually execute sell-to-cover.