It depends alot in the time period your talking about, the late 90's was I think an era were rapid evolution was happening at HW, there was uncertainy still about what worked and what didn't plus the sport as a whole was quite disorganized but by the early 00's I think you'd seen an influx of superior talent and training/tactics that had really adapted to MMA.
I think alot of modern training isnt an evolution of what someone like Fedor was learning its a devolution, Fedor was showing timing and defence way beyond what I think we see at HW today and mixing in some advanced technique such as hand traps. The kind of standardised MMA training thats become dominant in the current era I think is actually quite simplified in alot of ways, its aimed at rounding out the skill sets of fighters who don't always have natural ability in other areas.
In Fedors case I think he was a quality Judoka who had very strong natural boxing ability which was rounded out and adapted to MMA very nicely. He was working full time with quality boxing coaches and pretty regularly with quality kickboxing coaches.