I think it's pretty certain that under Chevron and w/o W. Virginia vs. EPA the EPA could ban ICEs tomorrow and the courts would allow it, and Congress would be unable to stop it, but that would be a very drastic thing to do and it should require debate, and we wouldn't get it. If you don't think that scenario is likely, there are others somewhat less extreme and more likely ones one could conjure up.
You can't pitch a hypothetical and say that since Congress wouldn't act in your hypothetical that they have in the real world abdicated their responsibility. I can equally make a hypothetical that says the EPA won't ban ICEs, or a hypothetical that says that EPA bans ICEs and Congress passes a law 24 hours later undoing the ban.
I think that's abdication.