But the unemployment that those workers receive is an externality. Now, the nature of UI does mean that it should wash out in the end, but in the near term laying off thousands of people simultaneously potentially puts a strain on the system.
Externality has a specific meaning. It is a cost of production passed on to the customer or third party.
Not everything that has a consequence is an externality.
If I put my trash in your bin, that would be an externality.
Not employing someone is neutral.
Laid off workers and the state are not harmed by the company that stops sending paychecks.
To think otherwise leads to absurd conclusions.