I would tell them where to stick their severance package.
Why not just take the money and then be “unavailable” when/if they called asking for help? The agreement is probably unenforceable in any case.
That said, this is another excellent example of the H1B visa sham: if we’re so in need of tech workers, why are companies laying off American IT people and then hiring H1B visa holders? Time to cut off ALL H1B visas for a few years.