This might make sense if they were still enrolled in the company email system, and there was some period where it had been communicated that there was some was

This might make sense if they were still enrolled in the company email system, and there was some period where it had been communicated that there was some was some expectation that they'd make themselves available for contingencies. If they're locked out of the company's email system and they'd been told 'this is your last working day,' and they didn't check their personal email/answer the phone ("whoops! must've gone to the spam folder!"), I don't think Twitter has a leg to stand on when it comes to forcing them to come back on the threat of lost severance. Their last contact with the company told them they didn't work there. It's pretty black and white. Just all around sloppy lawyering on that separation notice, imho.