Not sure quite what the fluff was in that era now but as Xenos also appeared it probably wasnāt pure HH though I think itās been retconned as such since.
Just want to add my $0.02 to that...
Epic started out as Adeptus Titanicus in 1988, which was titans vs titans during the Horus Heresy (sound familiar?). Then Space Marine was released in 1989, this was marine on marine during the Horus Heresy. Eldar & Orks (and some Imperium stuff) were added to Adeptus Titanicus via an expansion called Codex Titanicus (basically a compendium of White Dwarf articles, the Eldar were in WD #110 for example, as was the way back then). Orks and Eldar also have a few pages in the background section of the Space Marine 1st edition rulebook.
No retconning has been done as far as I can tell, having looked into 1st edition Epic when I was looking for an edition to start with for my now stalled Epic collection idea, it's all Horus Heresy for 1st edition Epic.
Space Marine 2nd edition (1991), technically Epic 2nd edition was when it was moved into the 40k era with all the xenos/Chaos etc. of the time present, albeit again via expansions, although Orks, Eldar and titans were in the basic rules. Titans were further expanded on with 1994's Titan Legions.
So, looking at it, Legions Imperialis is technically a reboot/rehash of Epic Space Marine 1st ed, plus titans/knights etc. from the Adeptus Titanicus reboot (and they have new rules apparently), but without Orks/Eldar or any other xenos, no retconning though, GW just seem to want to keep it focused solely on imperial vs traitor.
Sources -
https://wh40k.lexicanum.com/wiki/Space_Marine_(1st_Edition)
wh40k.lexicanum.com
wh40k.lexicanum.com
wh40k.lexicanum.com
Edited for clarification.