Yes, that would negate his claimed concern if you actually close. And if you don't? He still gave the employees 60 days notice so he'd need to replace them regardless.
That was my point. He has to give the notice without knowing what you'll ultimately want to do after completing the due diligence. If you do want to go through with it, great, everyone wins. If you don't, though, you walk away unscathed while he has to scramble to find employees. By changing the contract, he's taking away your option of cancelling without a significant hassle.
edit: If that were the only reason for his stipulation, you could suggest delaying closing to allow for those 60 days. In other words, take the usual 60 days (or less if you need) to complete due diligence and secure financing, then have him give notice once you are convinced you want the park. But since he also uses "not wanting to waste his time" as a reason, that option won't work either. Him using those two excuses backs you into a corner.