I think he defeated an admiral because that's the best way to etablish as the top dog of his era. Even the weakest admiral is still a top tier while for HKs we got the likes of Sommers. Maybe he defeated an Elder though
I don't think Xebec is in need to be established as top tier.
He already has massive hype
"Roger's strongest foe"
"Godvalley incident"
"Having monsters like WB, BM and Kaido as underlings"
All points to him being the top dog of his time.
If oda really wants to boost his hype then best way is to have him defeat an Elder or someone like Garling - Former captain of HK and present elder of WG.
As admirals doesn't fit in elbaph immortality talks. Infact, just few chapters back we got the lore of CoC and immortality.