Exotic fusion in the cores of gas giants?

Quoting from Wikipedia:
'Saturn has a hot interior, reaching 11,700 °C (21,100 °F) at its core, and radiates 2.5 times more energy into space than it receives from the Sun.'
Once upon a time Arthur C Clarke wrote 'The Sentinel', and then '2001: A Space Odyssey'.  Then later he wrote '2010: Odyssey Two' where Jupiter was converted into a star.  It is difficult to say to what extent the
words 'brown dwarf' were in common usage when it was written.
So Wikipedia talks about diamonds settling or something like that to
radiate 2.5 times more energy but.
There is geologic time.  This is enough for billions of years?
The Earth it is generally accepted has something called a 'mantle'
and something called a 'core'.  Nonetheless if you have ultra-compressed
hydrogen, or even ultra-compressed something like carbon, nitrogen, or
oxygen (compressed so much that is it even obvious that the molecular
bonds would still be something like, 'methane', 'ammonia', or 'water'), is it really so obvious that something like an exotic form of 'fusion'
is impossible at a very slow rate?  If it were possible, then would
it be impossible that there could be a rethinking of the term 'brown
dwarf'?  The Earth is supposed to have a 'mantle' and a 'core' - something like semi-'liquid' 'rock' and then hot 'metal', but mostly
compressed 'hydrogen'?  What does it mean?  Is it wrong to call than
an 'ocean' rather than a 'mantle' if it is something like a compressed
'gas'?