"Just curious as to why the NCSC in the UK spoke up so rapidly in support of US corporates rather than simply denying knowledge about what had, until that point, been a Chinese/US issue?"
Splitting that into two -
Why they spoke up so rapidly? Maybe someone in the media asked them for a response.
Why they gave the answer whey did? Because it smelled as wrong to them as it seems to have done to most others with a clue.