Flat pack stuff has been around much longer then IKEA. The real wood stuff was great, but heavy and inconvenient to transport. That's why the flat pack stuff caught on so fast.
Particle board is heavier than un-processed wood. Ikea does sell some stuff made from actual wood if you look for it. I bought an unfinished pine table from them for $60 a few years ago.
It really depends on the variety and the wood. MDF or medium density fiberboard is quite dense and heavier than say, pine wood. But not all particleboard is the same. There are many kinds and they vary greatly in density and composition.
I’m not sure what being heavier is supposed to mean though. Heavier doesn’t mean stronger.
I also said dense. Particle board comes in many densities (as does wood). The range of both of them overlaps, so there is no way to make a sweeping comment that one is more dense than the other.
Well you made a very strong statement to someone: “that’s just false.” And you were not correct. Correcting that with actual nuanced information is not being pedantic.