That was the reason in Rogue Trader, yeah but it got dropped super fast when GW
solidified the difference between the Legions and the Chapters.
This is slightly wrong: Primarchs weren’t even mentioned in the Rogue Trader rulebook, and the terms Legion and Chapter were used interchangeably. The first edition of Epic (or ‘Space Marine’ as it was unimaginatively titled) introduced Horus and Guilliman as characters (and Leman Russ appeared in WD
fluff about the same time) but simply as Imperial Generals, not the genetic-magic supermen we now know. Per Space Marine 1E three legions were deleted from the records… but nowhere was it stated there were only twenty; read at the time, the tone of the fluff in the article on Andy Chambers’ Valedictors in WD
136 seemed to presuppose there were as many legions c30k as chapters in 40k
. Twenty legions didn’t become an established background element until some WD
articles (essentially early versions of the fluff from Codex: Ultramarines) were published just before 40k
2E was released.