I'm afraid that is incorrect. I own many disco ones, and the bass is what they're for. The grooves are spread out and it frequently runs at 45 rpm. It's pretty obvious if you listen to one vs the 33 version.
The non-existence of ads on the internet from 40 years ago is not a validation of your universal claim. Second, people still press 12" 45 rpm singles, why would they do that when 33 rpm is available?
My 12" single of "Ipanema" from the B-52s is 3:58. The album version is 4:24.
My 12" single of "I'm So Excited" by the Pointer Sisters is 3:47. The album version is 3:53.
I doubt that'll convince you, either. But I recommend that you actually obtain a 12" disco single from the late 70's and a 45 and compare with your own ears. It's also fun to look at the difference in the grooves with a magnifying glass.