To some extent, the answer depends on how you are defining your terms. My interpretation of the docs is that it's okay to:
- Sell Steam keys through other venues provided you are doing so in a way that's generally comparable to how you sell on Steam
- Games using Steam keys can use the Steam infrastructure (servers, bandwidth, etc), even if those keys were not sold directly through Steam.
I was not able to find definitive info (edits to the contrary are welcome) addressing whether or not, you cannot make use of the Steam infrastructure without a Steam key. I could see this being allowed for things like cross-platform play, but wasn't able to determine if this was being done on Steam or 3rd party servers. On the other hand, it seems reasonable to assume that Steam doesn't want to bleed out money or assume legal liability for content completely outside their ecosystem. They pitch use of their infrastructure as way of "enhancing the value of legitimate copies."
If the documentation doesn't address your concerns, I suggest reaching out directly to Steam for additional clarification.
Here is an excerpt taken from the public facing documentation on Steam keys:
Steam keys are meant to be a convenient tool for game
developers to sell their game on other stores and at retail. Steam
keys are free and can be activated by customers on Steam to grant a
license to a product.
Valve provides the same free bandwidth and services to customers
activating a Steam key that it provides to customers buying a license
on Steam. We ask you to treat Steam customers no worse than customers
buying Steam keys outside of Steam. While there is no fee to generate
keys on Steam, we ask that partners use the service judiciously.
Steam Key Rules and Guidelines
You should use keys to sell your game
on other stores in a similar way to how you sell your game on Steam.
It is important that you don't give Steam customers a worse deal.
It's OK to run a discount on different stores at different times as
long as you plan to give a comparable offer to Steam customers within
a reasonable amount of time.
Here is an excerpt taken from the public facing documentation on Steam DRM:
The Steam DRM wrapper is an important part of Steam platform
because it verifies game ownership and ensures that Steamworks
features work properly by launching Steam before launching the game.
The Steam DRM wrapper by itself is not is not a anti-piracy solution.
The Steam DRM wrapper protects against extremely casual piracy (i.e.
copying all game files to another computer) and has some obfuscation,
but it is easily removed by a motivated attacker.
We suggest enhancing the value of legitimate copies of your game by
using Steamworks features which won't work on non-legitimate copies
(e.g. online multiplayer, achievements, leaderboards, trading cards,