> Assuming carbon based life as we know it, 300-400C is the probably a hard limit at which point single carbon bonds begin to break down.

Carbon-Carbon covalent bonds are the strongest we know. 300°C is nothing, carbon melts beyond 3000°C. Some organic molecules in our cells degrade at higher temperatures, but it depends on the availability of other species to react. Otherwise, hydrocarbon chemistry can be done beyond 300°C.

I am not saying that this kind of organisms are likely (I have no clue, it’s not my field). But from a chemical point of view there are 2 factors that could be at play:

- exotic organisms could rely on different chemistries altogether to produce energy and rely on molecules that are unstable under our standard conditions;

- and also chemistry under pressure could be very different from what we are used to on the surface.

I don’t know why you think carbon’s melting point or hydrocarbons are relevant to basic biochemistry. The “strongest” bond we know of breaks down starting at 300C (~500C for double iirc). It’s not bulk graphite or under extreme pressure underground.

You can fantasize about exotic biochemistries all you want, but what we know about carbon based life here on earth is rather specific in its limits.