The real problem here is the lack of documentation though - like, if you use non-standard tooling or style or decisions, the burden should be on you to document them. Sounds like in this case this hasn't happened - which is really sad, but not much different from awful (but standards-conforming) codebases with zero documentation.
If he actually documented his abbreviations, mnemonics, style and architecture, you would have most likely appreciated it instead of cursing it, I think.
This is true, but if he had documented anything then he would have become significantly more replaceable.
So replaceable in fact that the company wouldn't have had to hire me at great expense and could have got a normal developer in to take over and continue.
Maybe I'm just extremely jaded since my job is fixing messes left by people with more style than sense but I see striving for uniqueness to be a flaw in an engineer.
Fair enough. But for him (and you) it worked out great! He got some job security, and you got a job by fixing the mess. If there wasn't a mess, you wouldn't have this job:)
If he actually documented his abbreviations, mnemonics, style and architecture, you would have most likely appreciated it instead of cursing it, I think.