FFX was the prequel to this, Really?

[Sephiroth]

IMO it just makes it easier to pretend that FF10-2 doesn't exist.

If I have to consider it valid, then I would say it is absolutely deplorable. It smells like bad fanfiction writing, trying to shoehorn in as many connections as possible to stamp it's mark. But that's what I think of the majority of the Compilation.

This is worse than bad fanfiction writing in my opinion. I've never played FFX but I have seen it before, and it has very little in common with FFVII. There's no materia, summons don't work the same, there's no real lifestream, it contradicts the fact that Shin-ra never actually made it to space because they found mako to be profitable after their prototype rocket never made it off the ground, and in general it's just plain incompatible with FFVII because if you think about it really hard, there's just too many differences and too many plot holes which can't just be handwaved.

It has all in common as it is canonical to each other within one universe and Spheres and Materia are pretty much the same thing as well as the substance of Farplane and Lifestream are the same. A creator stating an idea that directly but still just calling it an "idea" does not automatically mean it is just an idea (even though being nitpicky other possibilities would still be given) but a statement of something as it actually is, just like Dumbledore's sexuality is a fact even though it was stated as "thought of" instead of "he is". In fact many creators state their facts like that and as long as it does not get to much into theory such as "probably, probably, probably" that is okay. And the study of planet life fits very well to Spira as well as Dirge of Cerberus' explanation of Omega's journey through the universe to give life to a planet. Final Fantasy X is seperate story-wise but in the same universe. The idea is just not totally fleshed out.

It's just a playful kind of nod, but I don't think in regards to the story it has any truth to it.

Exactly the opposite is the case and no one said the Gaians knew anything about the Spiran life before. Canon is canon, it really is that simple. Zidane's comment is a gag but this connection about the story was made intentionally.