Random observation really, not really thought about it too much.
It seems like they always end up specialising and advancing in one part of it, and in a kinda theoretical aspect.
SG - VI
RSV3 - Subtypes
JohnDo/Che (whatever name is) - DCNH
Gulenko - Subtypes, typing just about all celebs at ENFj or ENTp
Tcaud - Dual type theory
I can't really think of other types that do this sort of more theoretical, maybe "abstract" specialising.
But it seems that most other people struggle to get to grips with what they are coming from. VI probably works but no ones really been able to pick up the skill which SG uses it, same with DCNH (making it into face shapes and other stuff), and subtype strength for RSV3 - I don't really meet people where I can classify their subtype strength to any great usefullness for myself when interacting with them. Dual type theory is interesting but I can't really seem to understand how it's worked out.
Dunno about the other INTj's, not even sure if it's thread worthy or correct but thought i'd mention it cause i'm bored. Maybe other INTj's end up doing something like this with some other of their interest.
Seems on the face of it a bit eccentric direction to me, and I end up not really understanding it.