Modem manufacturers most definitely do have a role to play in that. Not all modems are created equal, and if Apple sold their modem to other companies they would have to provide a datasheet listing sensitivities at different bands.

That's to say, it's unlikely that Apple is the sole reason that this guy only gets 4 bars, but if you compared an iPhone and a Galaxy or Pixel at the same place at the same time you would get 3 different signal strengths.