You may be right. I misread the line a bit.
"As of TODAY, the Pixel and Pixel XL are the only phones that can use Google Assistant outside of the Allo app."
I'm just reading between the lines here. The Pixels will be the first phones to get 7.1... and Google said that the 5X and 6P will get it by the end of the year, with one or more
developer previews prior. That seems somewhat odd to me to be honest, since if 7.1 releases with the Pixels, why will it be in developer preview shortly after? The QA person in me comes to the conclusion that 7.1, even 'released' is still a bit of a work in progress.... as is Google Assistant. Now, when will the 5X and 6P get more of Google Assistant? That remains to be seen... but I suspect we'll get a subset of GAsst along those 7.1 builds with a more integrated GAsst when 7.1 releases 'later in the year'....
Later in the year sounds ominous, except when you look at the fact that people will get their Pixels about the end of October, so the end of the year is but two months away. So figure a DP build beginning of November with a full release of 7.1 in December.... then maybe a more phased rollout of more GAsst features. Then once the non-Google sourced phones get 7.0/7.1... whenever the OEMs get off their backsides and get it out, then there will be Google Assistant goodness for all.
What will Google Assistant be on the Nexus (and OEM) phones? I don't think it will necessarily be the same as the Pixel, Google might keep some goodies held back. But I think that what we get of Google Assistant will be a plug in place replacement of Google Now and Google On Tap. What we DON'T know is how much more that Google Assistant is on the Pixels.
Why so optimistic? Google never said "Phone-level Google Assistant will always be a Pixel exclusive feature". That would be both counter-productive AND self-destructive. Google wants everyone... iPhone users included, to use their services. Cutting what is becoming their biggest, most important service off at the knees would pretty much assure its doom. It may very well be that the Nexus and Non-Nexus phones get the same Google Assistant.
But that's exactly what we get now. The only difference between how a Nexus phone has Google Now and On Tap versus what LG or Samsung have is that the Google Launcher is pre-loaded as the default system launcher. Functionality wise, everyone has access to Now, has access to the On Tap features, etc. So if those get replaced by Google Assistant, but without some as-yet-unknown or unmentioned Pixel exclusive bits....