I read that comment from
@Leatherboot69 as a reference to what PADI says in its blog about its history...
Not so much that PADI changed something internally to make it more modular, but basically the difference between the PADI system and what was previously then common training from NAUI, BSAC and others which seems to have been more classroom time, more theory, more "linear" instruction.
All way before my scuba time (and I was still a pre-teen in 1978), and possibly I am misunderstanding either leatherboot's comment or yours - but that was how I took it...