Here is the relevant passage (AD&D DMG, p 40):
...
Magic absolutely had a source in AD&D!
Thanks for that quote! I never had the 1e core books (had some fun supplements though), so I hadn't seen that particular explanation. I have to give credit to Gygax for actually explaining it. As far as I can recall (and correct me if I'm wrong) there hasn't been that level of detail since. The 5e description is, as far as I can tell, the only serious attempt to define how magic works in the game overall since that 1e description.
The Weave is very specific, and quite different from the AD&D lore that I just quoted. From the Basic PDF (p 81):
...
That's quite different from what Gygax wrote in his DMG.
I'll give you that it is different, but I don't think it is insurmountably different. Primarily, the parts of magic that the 1e DMG describes are mostly how spellcasting works. The 5e description of spellcasting basically just says that the arcane caster directly plucks at the threads of the magic fabric, while divine casters have their access mediated by another force. It isn't too much of a stretch to say that the Positive/Negative infused magical words are exactly what it is that allows the manipulation of the weave, and that arcane casters use their own skill to do so, while divine casters have the energy and capacity to do so intuitively grasped (mediated) by their divine communion.
I'm not saying that 5e version attempted to make them compatible. I'm just saying that it can be done via interpretation. It doesn't require abandoning either version.
Just for starters, it locates magic in the world itself ("the stuff of creation") rather than in other planes; and the caster manipulates magic (via the weave) rather than channeling energy from another plane. That's real change. It's certainly not clarification of what Gygax wrote.
And this is where they are still compatible! 5e actually does relate magic to the planes. It says it infused the entire multiverse. Most of the multivere is other planes!
One could easily say that Gygax's version is an actual in-world interpretation (perhaps on Greyhawk) that describes the universal magical interface posited in 5e as the planar energies.
Again, this is a matter of interpretation, not of contradiction. We could easily be having this discussion in-character and it would be a perfectly acceptable academic discussion between Elminster and Mordenkainen concerning whether magic inherently worked differently on their respective worlds, or if they were simply viewing it differently.
That, to me, is a successful (and interesting!) level of compatibility.
By contrast, the 5e MM gives a new backstory for the Yugoloths which outright disagrees with the prior one. The only way I can even let it stand as in-character lore is to say that is the lie that the Baatezu like to tell people to make themselves seem cooler than the Yugoloths. It is a real contradiction.
Amendment:
In Dragon #103 (1985) Gygax had an editorial saying it's time for TSR's writers "to start seriously considering a second edition", in which he currently plans "Mystic, savant and jester to be introduced as new official classes".
And that is why, unfortunately, Gygax was sometimes just off his rocker. A core PHB
jester class? :rollseyes: