Yeah, notice periods are typical at that level even in the USA where they're not often used for the rank and file. A star candidate may have had sufficient leverage to negotiate them away, but I don't expect that to have been the case here.

Sometimes a new employer will offer compensation for any loss of stock options and/or to indemnify against any claim for breach of contract or similar.

But it could also be that Mongo have simply agreed to release him - he was only there for 10 months, and might not have turned out to be a great fit. Their share price has been more or less level in that time whilst competitors have been rising so it's not as if he had notable immediate success in boosting their appeal to enterprise customers.