It doesn't sound like Kyle knew there was a claim. He probably just assumed since he had a vesting schedule which is standard so would anyone else. "only came up after deal announced"....
No, I mean I think maybe GM's legal team should have figured out about this situation before the merger was publicly announcement. Sure, maybe no-one at Cruise would have thought to bring it up, but I'd expect that the due diligence process might be thorough enough to catch it.
If the Cruise team didn't even know it was a problem- no way for GM to discover it. If the corporate entity is unaware- there would be nothing for GM to discover.
Obviously probably regrets that assumption now.