You might have skipped the introduction, but the pseudo-assembler is MIX assembly language, and the hypothetical mythical machine is a central part of TAoCP. The main idea is to show how a computer works at the machine level and implement everything from the ground up, using first principles and mathematical rigor. You are right that the mythical machine itself got old, which is why it was superseded by MMIX.

So no, it’s not a blunder by DEK to avoid abstracting the complexity into higher-level languages, nor to have not written it as a handbook. TAoCP is not a handbook-like book, it is well beyond that. There are plenty of books for that. For that purposes, you might consider consulting CLRS or Skiena instead.