If an apporoach is labelled 'CAT B only'; any aircraft can fly it, provided they'll remain at or below 135KIAS (or, say, 130KIAS for the MAPt).
Definitely not true and potentially quite a dangerous idea to try to circulate. It demonstrates a complete ignorance of Pans Ops Volume 1, if nothing else. It doesn't matter what speeds any particular aircraft can
achieve, if below any of the speeds for the nominal category.
I know that the Dash-8, for example, is a very flexible aeroplane. No doubt about that.
But lets take the example of a very flexible type of aircraft that is nominally in Cat C. It doesn't matter if it can maintain 130 KIAS or below, from the FAF to the MAPt. It also doesn't matter if it can maintain 130 KIAS or below for circling. This is because the determining factor for this aeroplane is the basic 1.3 Vs at max certificated weight.
The same sort of consideration applies to all other aircraft in all nominal categories. Thus, getting back to the Dash-8, it is a Cat B aeroplane, regardless of what aspects of Cat A it can achieve.