That's effectively how it ends up, except with a slight advantage to the company. They cut you off and tell you that you don't have to work anymore, but in the off chance you get a job within 60 days, they don't have to keep paying you. They can also preserve their cashflow by not paying you up front.
But since you technically have to be "on the books", if something like Slack is tied to your status in the company directory, it's easier to just leave it.
Just an FYI - you can still get paid the WARN severance even if you take another job, just don't "quit" during warn. Your employment contract may or may not say you can't do side work, but (1) what, will they fire you? (2) it probably just says that you can't do work that interferes with your current employment, which is not a problem.
The WARN period exists to give you the money, but also keep you on for insurance and 401k vesting purposes (and similar). Getting cut off immediately, and suddenly losing insurance would be much much more disruptive, even with COBRA.
But since you technically have to be "on the books", if something like Slack is tied to your status in the company directory, it's easier to just leave it.