Why is it relevant at all that these were short-term rentals? Wouldn't these units have been just as deadly even if they weren't, since the real problem was lack of exterior windows?

A normal residential building wouldn't have windowless apartments.

The implication in this article is that a single apartment was split up into multiple units to make more money on Airbnb. The windowless unit was one of them, and the occupant couldn't get into a space with windows and escape.