One example to help think about this. Say you have 3 friends. Your friend Bob has a net worth of $1 - he is the least rich. Your friend Alex has a net worth $10 - he is richer. Another friend Ben has a net worth of $100 - he is the richest. Richest here is comparative against all 3 of them, but none of them are actually rich. Bill Gates is rich. Bezos is rich. Musk is rich. Someone with a net worth of $100 isn't.

You can still have comparisons between the rich too, so Bezos is richer than Gates and he's also the richest if you're just considering the pair. But add Musk to the mix, and he's no longer the richest.

I guess that last example looks like you have two attributes - rich as some objective "has a lot of money" and comparatively rich (richer, richest). For safe, it's kind of similar, except that as soon as you are saying one thing is safer than the other, then you are implicitly acknowledging that there are areas where the thing isn't safe, and if you're admitting that you can't also call it safe without contradicting yourself.

It's a bit ambiguous and depends on context, which is why I said 'at least in CS', since for whatever the particular topic is 'safe' and 'unsafe' is likely to have a fairly strict meaning.

In general you're right. For safety it's just that 'safest' implies some sort of practicality: the best - most safe - from a set of options. But the safest option isn't necessarily strictly safe.

(Say your dog's stuck on a roof on a windy day, you decide the safest option is scaffolding (safer than a ladder or free climbing), but it's not safe, you just insist on rescuing your dog.)

I would assume he refers to "safe" being absolutely safe, while "safest" refers to the safest of the existing alternatives?

Nah. If something is 'safe', it's safe, period. If something is safest is, it's only the best of the available options and not necessarily 'safe'.