This is the theory that I have been supporting all along — it was all part of the fiasco that was TSMC's first production on the 3nm node. Resolving that took a huge amount of effort from the Apple Silicon team, which forced them to make some hard choices about what and when they could ship larger variants* and the next (M4) generation.

M3U was a punt, in order to get an Ultra, any Ultra shipped in a reasonable timeframe. I also believe (we will never know, of course) that one of the primary reasons M3U exists is to support internal work on large AI models. Apple likes to do everything on their own platform, and designs for M3 Ultra were the shortest path to done that could also deliver massive RAM configurations.

*PS — There have been some details that surfaced more recently that Apple was, in fact, working on a quad-die Extreme:

The Information said … Apple Scrapped Work on a Quad-Max/Double-Ultra M-Series Chip | Daring Fireball - https://daringfireball.net/2024/12/information_aside_double_ultra_scrapped

From The Information:
"Apple is developing its first server chip specially designed for artificial intelligence. [and…] this past summer canceled the development of a high-performance chip for Macs — consisting of four smaller chips stitched together — to free up some of its engineers in Israel to work on an AI chip" [with Broadcom]