Just confirmed this.
Lua Code:
/run for idx = 1, 500000 do MountJournal.SummonRandomFavoriteButton.spellname:SetText(string.format("%d %d", idx, idx)) end
+10 Mb addon memory.
Lua Code:
/run for idx = 1, 500000 do MountJournal.SummonRandomFavoriteButton.spellname:SetFormattedText("%d %d", idx, idx) end
No changes to addon memory.