They might not want to go to Google. Doing this, an acquihire, they can at least get a couple years out of the team, holding the vesting of the shares over their heads, before the team breaks up.
That sounds terribly. You're hiring a team that wasn't able to deliver, for a ridiculous premium (essentially a ~$10m signup bonus per employee), knowing that long-term there's no structural future as they're leaving at the earliest opportunity, i.e. when you stop bribing them to stay on with a share incentive.
Perhaps they built some really promising IP that could work well with Google, in addition to an undoubtedly useful acqui-hire, that'd explain things. I don't think your reading is very plausible but who knows.
And this kind of short term thinking isn't uncommon for large companies. And by the time the vesting period is over, there will likely be newer, better technologies out there. And not everyone will leave after vesting, but you'll have to have done something to cause them to stay on before vesting is over.