Correct. But also, an antenna doesn't have to be exactly resonant at the desired frequency in order to help. You can see this most often in the antennas people use for HF, which to be resonant have to be giant and many meters in size, but instead they use small non-resonant antenna that still transmit enough, even at very low efficiency, to radiate an annoying amount of interference.

Not having a giant slot antenna is a lot better than having a slot antenna. Even if the slot antenna is best resonant at mid-VHF (ie, ~144 MHz which is what many GPU RAMs are blasting at at high current).

And just as important: a huge hole is a hole. Even if it's not acting as an efficient antenna. It's basically an entire side of the case missing.