I don't think it's true that "If someone has a Roth 457b plan, they're allowed to withdraw all the cash tax-free after they no longer work for the employer." While it is true that withdrawals are permitted after severence from employment", I don't see anything in the 457b rules that exempt anyone from the general rules for Designated Roth Account Distributions which state that any withdrawals that don't meet the requirements for qualified distributions are taxed.
It appears that the only distributions allowed from 457b plans while still employed (whether from designated roth contributions or not) are for unforseen emergencies. There doesn't appear to be any exception for designated Roth acounts.
Also there are big differences from non governmental 457 and government 457 plans.
Last, all of this is contingent on the fact that I didn't actually deep dive into the code or regs themselves.
Another link: https://www.irs.gov/retirement-plans/irc-457b-deferred-compensation-plans