What they mean by the “old gods” is actually the pantheon of gods that preceded the New Gods, it’s confusing due to the name but the New Gods actually are older than any of the earth bound gods(Greek, Roman, Norse, Egyptian, Celtic, etc), it’s a common misconception. When Darkseid killed the old gods he actually absorbed their power giving him the powers we know him to have today but the death of the old gods also created something called the godwave which not only created the pantheons of earth but of all other worlds too, and gave the emotional spectrum physical power that the guardians of course would go on to harness, it also technically crated the speed force although I though the act of lightning striking Barry Allen created the speed force so I’m not sure about that one. But regardless it seems that Darkseid has the ability to still absorb super powerful being’s powers (such as Zeus or Mazahs) and use them to replenish his own so it’s not far fetched to think he could do the same to Captain Marvel or Black Adam since they’re both super powerful too. We also see that he can use regular humans for the same purpose like we see in Dan Turpin in Final Crisis or Alexander Luthor Jr. in Darkseid War. In fact it seems that he more often uses lesser beings to attach himself to as an avatar that he can fully dominate and change into a weaker version of himself and then goes after someone more powerful, like Zeus or Lex Luthor literally imbued with Apokolipic power, to replenish his own power, kinda returning him to full health.
