He's arguing that rebuilding the internal structure of the roof with wood beams would inevitably lead to another fire. Concrete and steel would last longer and be more structurally stable and resilient to fires.
What he's not arguing for is changing the visual character of the building. The steel and concrete he proposes would be in internal areas that people don't see. The ceiling which is what people see from the inside would remain visually the same. So would the exterior. The part that would use concrete and steel would be the interior section of the roof which is only seen by maintenance crews.
Separately, he makes the argument that much of the stained glass installed during the 19th century was "severe and flat" rather than "delicate and jewel-like like genuine medieval glass". He's arguing that we should install glass more closely resembling the original medieval glass, which would allow more light into the structure.
I think all of this is very much in line with trying to keep the character of the building the same.
EDIT: Reading the end of the article again, it's actually not clear to me whether he is proposing to put in clear glass or a stained glass that's a bit lighter than what was there before the fire. I think I initially misinterpreted and the former is what he's getting at.