I think him working on the original geneseed project is part of the ‘misremembered myth’ angle. He wasn’t old enough to have lived on Terra during the Unfication Wars in Wolfsbane. People in 40k think he might have, but he actually didn’t.
According to Codex: Adeptus Mechanicus he probably is old enough to have been around for the initial Primarch and Space Marine projects.
From page 38 of Codex: Adeptus Mechanicus
"Archmagos Dominus Belisarius Cawl was already old at the birth of the Imperium, over
ten thousand years ago. During that span of
ages the Tech-Priest has served Mars and the
Machine God as a Forge Lord and a Lexico
Arcanus, but it is in his role as Magos Biologis
that Cawl’s greatest mastery lies."