Whilst this no stretching thing might be true for seasoned runners i.e. those for whom it is their main activity, those who have spent a lot of their life cooped up on a bike will certainly need some conditioning before running properly. Not to mention going from a low impact sport to one of the highest impact ones.
The latter reason is why starting off running off-road is good advice. However, you will need to lengthen those hamstrings and calfs, and stretch those tendons, so to say “don’t stretch” is pretty dangerous.
Having said that, I am an advocate of dynamic warm-ups, that help stretch by their very nature, whilst still being relatively low-impact.