Let me start with this paragraph where Hubbard speaks of "imaginative fiction":
When you mix science fiction with fantasy you do not have a pure
genre. The two are, to a professional, separate genres. I notice today
there is a tendency to mingle them and then excuse the result by
calling it “imaginative fiction.” Actually they don’t mix well:
science fiction, to be credible, has to be based on some degree of
plausibility; fantasy gives you no limits at all. [...] They are
simply very different genres from a professional viewpoint.
And a little before that:
So anyone seeking to say that science fiction is a branch of fantasy
or an extension of it is unfortunately colliding with a time-honored
professional usage of terms. This is an age of mixed genres.
That is why Hubbard spends so much time talking about fantasy. He's trying to distinguish science fiction from fantasy (hereafter SF and F) because he thinks there's a trend towards mixing genres and considering them SF. In his view, "pure" SF is any SF that doesn't have F elements in it. Towards the end, he says as much:
And as an old pro I assure you that it is pure science fiction. No
fantasy. [...] Science is for people. And so is science fiction.
Other than that, best that I can tell, he seems satisfied by the dictionary definitions:
So, by dictionary definition and a lot of discussions with Campbell
and fellow writers of that time, science fiction has to do with the
material universe and sciences; these can include economics,
sociology, medicine, and suchlike, all of which have a material base.
Note that, while he does say SF "has to be based on some degree of plausibility", that's a very subjective judgement, and since this definition doesn't use it, I think we can leave plausibility out of the equation.
And for F:
So fantasy could be called any fiction that takes up elements such as
spiritualism, mythology, magic, divination, the supernatural, and so
Hubbard also stresses on science and SF being about people, but this isn't about defining SF. Instead it's about the point of science and SF:
[It] was no use to just send machines out for the sake of machines,
[and] that there was no point in going into space unless the mission
had something to do with people [...]
The point here is not that SF that's not about people isn't SF, but that such fiction has no point (just as science that doesn't have anything to do with people isn't not-science).
To summarise, as far as I can tell, Hubbard thinks that "pure" SF is fiction that has to do with material universe and sciences see 1 above and does not have elements of spiritualism, magic and other non-"material" phenomena see 2.