I can’t speak to any physical limitations that this poster seems to be speaking to (I only studied physics theory so I’m not too sharp on the details of lab / experimental devices / apparatuses) but I would guess intuitively that a fundamental limitation is that radio tends to be more coherent, so you aren’t relying on individual photons but rather aggregating a bunch of photons to measure a wave. In contrast, optical light tends to be incoherent (unless from a laser), so you have to measure individual photons and they aren’t really correlated with each other (so interferometry doesn’t work).