I'm not saying I would or wouldn't take this particular buyout, but as you point out, it's not a safe assumption the default severance would be the same absent this artificially sweetened offer. At a quick glance, New Jersey is the only US state that requires any severance at all, beyond making employees whole for time worked and paying out accrued PTO. Many do have additional requirements around mass layoffs, but nothing comes anywhere near two weeks per year of service. Even NJ, as the best-of-the-best (honestly surprised it's not CA or MA, but not in this case), only requires one week per year of employment.
That said, I see the GP's argument come up quite often in any online discussion of layoffs, and it took me years to finally figure out, this topic basically comes down to a simple class issue: If someone is a moderately well-paid salaried employee, a severance offer is almost always going to beat UI by miles (and as you also pointed out, that's not a mutually exclusive choice, UI just picks up after severance ends). If someone is a low-paid nonexempt worker, by contrast, severance is usually nonexistent and employers playing games around UI claim appeals are a lot more common.
There's also other benefits to consider, most importantly health insurance. Severance offers usually allow employees to keep their group health insurance during the severance period, while COBRA is eye-wateringly expensive even for someone formerly paid well.
