It can really be argued...
It can't really be argued that the function was somehow separately provided
Of course it can. Lawyers can argue anything. And in this case they're right. The DNS is replaceable; it's what we used to call a value added service. 8.8.8.8 tells me how to find 2a03:2880:f119:8083:face:b00c:0:25de by name instead of by number. If the DNS vanished overnight and was replaced by the new Yes Me Directory service, we'd be fine (and I'd be rich, which is whole point of having a monopoly and exactly why the incumbents are on the FCC side in this lawsuit, and why ATT and the European PTTs were against the original deregulation efforts).
I'm all for network neutrality in a reasonable form, but actually the FCC is correct - it's a consumer protection and free competition matter, and it should be regulated under consumer protection and anti-trust rules. Not that that's good news for consumers in Trumpland.