I think everyone agrees that is the case, we are mostly wondering why that is the case. There are explanations, but the best explanations all cause weird things to happen with the current paradigm. Like, if Nerull is the concept of undeath, and beyond Orcus... Orcus would make sense as a lieutenant of Nerull, even if he was a disloyal and traitorous one. Instead, they have no connection except both of them being a source of undead, and unique undead at that.
The changing nature of both the concept of the multiverse and the gods makes things more complicated. In 3E and 4E, different settings existed in different cosmologies and had their own sets of planes and gods. 5E went back to the idea of their being a shared multiverse where all campaign settings take place in the same multiverse, but the gods of the Outer Planes are for whatever reason more strongly linked to certain worlds than others. In 4E's take, for example, killing a god was a bigger deal because it was the only god of that concept in a given setting's multiverse, whereas in 5E there are many gods of the same kind sharing the same Great Wheel but generally being limited to actually being worshiped and having power over certain worlds.
Taking Nerull in particular, he is most strongly linked to the Greyhawk setting. In both the default Points of Light setting of 4E and Critical Role's Exandria Nerull was killed by the mortal mage who would become the Raven Queen after stealing his divine power over death. In 5E the official stance as of Mordenkainen's Tome of Foes was that the Raven Queen had a different origin and nature, was only worshipped as a god in some settings, and that she did not kill Nerull. Then Explorer's Guide to Wildemount was released and pretty much reverted her back to her 4E lore.
Taking all this into account, it seems that Orcus would benefit the most from becoming a god in a 3E or 4E cosmological model where he would be the one, undisputed God of Undeath of the setting. In cosmological models like 5E's, becoming a god seemingly has more cons in that gods are strongly linked to a small number of worlds, perhaps even only one. With that in mind, would Orcus rather be the Demon Lord of Undeath that can operate in multiple campaign settings, or would he rather become a God of Undeath in only one world of many? Further, why would he serve Nerull, who is only a god in Greyhawk? These and other details personally lead me to continue to use the 3E/4E take of settings being isolated rather than part of a broad multiverse of settings.
While I'm writing a wall of text, I would also like to point out that the identity of a being can impact its flavor and what else is associated with it. A god of undeath presumably has a realm among the Outer Planes and relations with other deities. Orcus has both undead and demonic underlings as well as rival demon lords, such as Demogorgon, who all want to be the most free being in the multiverse with no one to answer to. Atropus, the World Born Dead, is a moon-sized undead Elder Evil of mysterious origins that seemingly has little motivation on its own and only appears to create undead when drawn to worlds that have experienced death on a massive scale. Rather than have them all linked, these different entities could oppose each other, or Orcus could try to seize control of Atropus or something.