A company paying half a million annually to ensure this employee is retained. It's not meant for joe sixpack making $100k/yr as an underpaid consultant.
Signing bonuses almost universally have a 1-year clawback (or are otherwise only doled out periodically and not all up front), so not a good analogy here.
They had no problem offering 7-figure salaries to PhDs with research experience in AI a few years ago. Those are the exceptional workers the program was supposed to be bringing in the first place, not dime-a-dozen JS vibe coders.