Just because it's an interesting and fun thing to debate, I'll ramble...
I think intent matters more than the method of distribution, so Lynch/Frost writing it as a large chunk and shooting it like a film matters more than the fact it was broken up for television (in my eyes). TV was the only way this could have been shown, so it's simply the nature of the beast. So more than anything, what matters is what it feels like, and the overall lack of cliffhangers up until the very final parts as well as the "random" sequencing/shaping of the nonetheless steadily building narrative ensure that all scenes feel perfectly natural coming one after the other, notably including closing scenes and opening scenes seamlessly transitioning from one Part to the next, as though credits weren't even supposed to be there. So, that it was written/shot like a film, made by a filmmaker who (crucially for this debate, imo) directed the entire thing, has very few cliffhangers or standard episodes or self-contained episodes...all of that speaks to its status as a film, to me, one made with less attention paid to the individual chapters and more to the overall whole. Even Part 8 starts with 15 minutes that continue straight on from Part 7 before becoming what most would refer to as a "standalone" episode, even though the Atomic Bomb sequence is triggered by the emergence of Bob from Cooper's body (quite similar to how the asides in a film like Kill Bill (anime and training sequences) occur directly in response to some stimulus in the protagonist's story). To me, that feels like a natural event at that point in the narrative, and does not signal episodic television (Plus, as evidenced by the Tarantino examples, some films do have chapters and are still definitely films). Similarly, Part 18 feels not like a standalone, but as the final hour of an 18 hour film. That's how it all feels to me, especially early on, when none of the episodes resemble standard episodes of TV and each seamlessly bleeds into the next. At the very least I don't see how anyone could look at Parts 1 - 8, what with so many seamless segues from Part to Part and a total lack of signified cliffhangers, and say that it doesn't totally resemble a film that was simply broken into hour long chunks, or that a Part like 12, which was hated by so many upon airing for containing so little forward movement, is anything but a collection of scenes rather than an actual episode of TV as we know it (yes, it's built around themes, but opaque thematic assembly is in my experience a new way of putting together an hour of TV, which actually eventually reveals itself to be the guiding principle behind all 18 hours of The Return as a whole, more about theme than narrative). But that's just the way it feels to me, 100% filmlike, a continuous dream that is meant to be watched in as short amount of time as possible so as not to disrupt the dream. So I suppose it comes down to how it feels to the individual viewer, how seamless it all feels. Would this thing have been edited differently if it were being shown only in theaters? I'm sure some of it would have been, and I'd love to see that version. But that it wasn't included on the Blu-ray speaks more to me that Lynch already spent so much time editing it one way that it would be too time consuming to edit it another. Either way, to me it's still a film...as well as a Series. As well as something else entirely. Which is all a long winded way of saying: It was shot with a camera by a filmmaker and yet it's unclassifiable, so I don't care what anyone calls it, just as long as they don't put restrictions on what it's called.
And as far as objectivity goes regarding Sight and Sound determining what is and isn't a film, the precedents are all the looooooong films broken into chapters aired on TV by master filmmakers that routinely make films lists, ie Scenes From a Marriage, Dekalog, Berlin Alexanderplatz, etc. Twin Peaks: The Return is definitely part of that lineage, which is why I find it so odd that there's even a debate.
And I agree with Xavi about the Roadhouse sequences. They're so integral.