Nothing. It means nothing. It is in the current CEO firing template, that is why it's there.

He said "I take full responsibility for the decisions that led us here"

You can argue about the semantics of "responsibility" means, but i think in context he's basically saying "my bad". Nothing more nothing less.

If you look at the dictionary definition https://www.merriam-webster.com/dictionary/responsible this way of using it is consistent. "being the cause or explanation" is one of the definitions. Sure there are other meanings to the word, but this is the one that makes the most sense in context even if you wish he meant the other one.

It means he will pay severance, benefits for months even after the people are gone.

It’s just that people are so used to this entitlement that they overlook it by default.

Wasn’t there a whole scene in Silicon Valley with Gavin Belson that basically mocks this kind of language?

I interpret it as “a decision was made, it was wrong and I take responsibility for that decision”.

The other option is he blames some external force or some other leader in the company.

Taking full responsibility here should have involved firing himself along the 12K. This is the norm in many cultures, resigning (or even killing yourself, sadly) when being responsible for a major failure.

The same, that it was a miatake on his part, even though he was getting signals from different people that Google is overhiring.

It means "I take the blame and some consequences thrown at me" .. that means yes I will do nothing just absorb the hit which is probably nothing.