It’s more than that. Tesla engineers themselves probably are itching to help. Check out GE’s workers getting mad that the company wasn’t helping to make ventilators. People join companies like Tesla and SpaceX because they want to be challenged, have a bias for action (of course, everything still needs to be qualified), and make the world a better place. In a global pandemic that can be helped by addressing a shortage of manufactured stuff, engineers aren’t going to want to just sit on their hands if there’s a chance their efforts could be helpful.

It’s not just corporate PR. It’s not just Elon Musk. It’s also these workers wanting to help the world.

Tesla (and SpaceX) has experience with life critical code (running vehicle powertrains independent of the rest of the vehicle subsystems; please save Autopilot critiques for another thread).

A fully validated ventilator is ideal, but if your other option is death, a ventilator that is "less than fully validated" might be preferable.

EDIT: You're right to acknowledge there might be bugs, it's important to quantify risk. You decision based on the risk(s) raised.