As to why, no idea. I guess some engineer was just familiar with it from their undergrad days like the rest of us.
And which version, I know it’s MINIX 3 but beyond that? No idea. They probably heavily modified it and as Minix is not GPL, Intel never published it. Based on the timelines it’s likely 3.1 as the ME platform has heen around since approximately 2007 iirc.
The original L4 was written in assembler and replaced by different other implementations long before the ME platform was developed. Pistachio was in development around that time and available under BSD.
AIUI Sel4 is just a kernel, so adding all the "management engine crap" - networking stacks, drivers etc. would be a lot of work. Minix came with 'batteries' included.
Which version of Minix did they actually use? There is Minix v3.1 (released in 2005 with the book), 3.2 (released in 2012) and 3.3 (released in 2014).