Incorrect. A mask rated N95 filters out more than 95% of particles (PM) larger than 0.3 microns — that’s much smaller even than PM2.5 microns. Certified 3M N95 rated masks are sold all over China.
The question is how much air bypasses the masks at the sides. The holes there are huge and any air that passes there will basically have the full smaller particle load. Do the masks produce considerably more resistance to breathing if you press them on with your hands? That would be a sign of this happening.
I question this; "pm2.5" is a big marketing term in China. Do you think the masks are lying about what they filter, or were you under the impression that people weren't buying pm2.5 masks?