The opinion that the burnt item is a section of the MH370 fuselage - and supposedly from the avionics bay - is purely the opinion of the finder, and not backed by any authority.
We have to recognise that Blaine Gibson is a man driven by a desire to find MH370 and wreckage from MH370 - and nearly every piece of flotsam or jetsam that he finds on Madagascan or Mauritian shores, must be from MH370.
I do admire Mr Gibsons single-mindedness to try and find MH370, and the reason/s behind its demise - but he is conjecturing to all the assembled headline-hungry media of the world - and probably the best thing he could do, and what he is apparently planning to do, is to take every suspect item he has found, directly to the ATSB for professional investigation and evaluation.
The explanation for the burn marks could be something relatively innocuous and simple - or it could be ominous.
Only expert air crash investigators have the ability and equipment to come up with a qualified opinion.