Do note that the this claim comes from the same people who don’t hand out any actual specs and always fly with an additional reflector. The latter makes it easy for radars to see them again. This is helpful in allied airspace, but it also makes it impossible to verify the claim.
Also note that modern radars are sensitive to how fast an object approaches (or leaves) the radar. Bumblebees don’t break the soundbarrier usually, so it is possible to see these planes, but you do have to tweak your radar for it. (Hence why the US doesn’t give specs.)