The difference between these theories is actually in how they interpret the idea of "how likely each consequence is to happen if it does this action or that"; hence they differ even in that "simple" case.

EDT does differ from CDT in that case (hence the Smoking Lesion problem), but EDT is clearly wrong to do so, and I can't think of any one-player games that CDT gets wrong, or in which CDT disagrees with TDT.

both simply regard things that we might classify as "other agents" simply as features of the environment.

I don't think this is right in general- I realized overnight that these decision theories are underspecified on how they treat other agents, so instead they should be regarded as classes of decision theories. There are some CDT agents who treat other agents as uncertain features of the environment, and some that treat them as pure unknowns that one has to find a Nash equilibrium for. Both satisfy the requirements of CDT, and they'll come to different answers sometimes.

(That is, if X doesn't have a very good ability to predict how Y works, then a "feature of the environment" CDT will treat Y's action with an ignorance prior which may be very different from any Nash equilibrium for Y, and X's decision might not be an equilibrium strategy. If X has the ability to predict Y well, and vice versa, then the two should be identical.)