It's the "if we end up disagreeing part" that seems like blackmail. And, hey, maybe that's what they're going for! Could say "yep, I want to be able to blackmail them if I need to." But the comment I replied to wasn't that explicit.
It’s leverage held in reserve, not planned for immediate exploit. If we part ways and all is hunky dory, it sits stale. But if e.g. the firm gets bought by an asshole and he frivolously pursues ex employees for dumb reasons, e.g. deciding a non-compete covers everything from finance to gardening, I have something to fight back with. (One can similarly ask why the company needs non-disparagement protection.)
Employers and former employees get in stupid tiffs all the time.
For a few million dollars for a bounded period of time, sure, disconnect yourself from the Internet for that bounded period of time. For $40k, you're just taking on unnecessary problems that you don't actually have the resources to solve.
(I was offered $0 to sign an NDA after leaving my last job. I did not sign it.)
I read sagas about people getting doxxed every day, even children. You may trust your opsec, but it only takes one slip-up to be out a large quantity of money. I wouldn't risk it.
How is it blackmail if the company is the one offering the money?