Absolutely. This is my point. The fact that SteamOS is Linux is basically irrelevant to publishers, just like Android being Linux is irrelevant to app developers.
Android uses the Linux kernel. But when one talks about Linux, one usually means GNU/Linux: that is, the kernel and the unix (or rather, not unix) userspace.
Android has little, if any, of that user space. That is, assume I have a dynamically linked Linux program: I'd probably won't be able to run it on Android, even if cross-compiled to the proper hardware, because much of the libraries and tools one expect a Linux distribution to have are missing there.
Because you don't run just a kernel. It's fairly obvious a lot more things go into an operating system. Calling Android and Ubuntu under the same "Linux" umbrella is like calling bread and pasta as "Wheat" since they use the same raw material.
if you want to go down this road Android !== GNU Linux || Android !== Linux as the first compare two OS, and the second compare an OS and a kernel. Sure they share a lot of libraries that are in use in those sort of application (more so than GNU Linux and Windows variant) but still, those are not the same.
Mind this is begging a question: with all the TV box running Android, are manufacturer going to target Android or SteamOS (More and more so called smart TV are running some version of Android)? And what about indie developer? Right now you can pick up an Android TV Box for anything in between 50 and 100 GBP.
That is, they may share a kernel and some tools, but the user spaces are mostly alien.