On the other hand, I've seen early modules that suggest modifying the opposition based on the number or level of the PC's.
Tomb of Horrors suggests modifying the PC's based on the number of players and their skills.
That definitely exists. None of this stuff was ever monolithic. Especially back in the 70s and 80s before we really had forums like this. I think it is a matter of how prevalent it was. In 1E, my overall memory was tailoring things less to party power level. However I do remember a lot of the things we are discussing (tailoring encounters, lower lethality,etc) creep in more and more during 2E. The first time I remember seeing advice to fudge rolls to save PCs was 2E (I believe). I am sure it came up prior to that. But at the same time, Gary talked about fudging die roll results (usually for other reasons) in the 1E GM guide, if I recall.