Eh, as I stated above, there's nothing "black and white" about those terms, especially as they're currently being applied. There's what the band considers their work to be, what others consider it to be, and what it really is-- and no one should put too much weight behind any of that. Remember how, for the longest time, Steve Wilson rejected the "progressive" label for his music?
Frankly, when Mike Portnoy says things like "No, AMBI is a suite, SDOIT is one song," he's doing so because he's trying distinguish something that should be listened to as a whole (SDOIT) with something that can be listened to as a whole or in parts (AMBI). To my ears, SDOIT is very much like a "suite" and AMBI is more like those types of song-cycles SDN mentioned above. Whether the band call it that or not is inconsequential. If the band were actually really that clear about defining what works are supposed to be considered what, and if the distinction between "suite" and "really long song with different movements and elements" was actually a meaningful one, I'd be more willing to admit that I was "objectively wrong" about that.
Until then, though, I'm not going to jump down anyone's back for calling Six Degrees a "suite" when it's supposed to be "one song." It is really like a suite in every way and, honestly, it works better as that than "one long song" anyway, where it's disjointed at best.
Out of curiosity, do you know how Neal Morse describes his songs? "Seeds of Gold," to me, seems to be a suite. As does the Whirlwind. But I'm interested in what he says. Is Whirlwind a suite, a concept record, or one long song? Those definitions aren't necessarily conflicting with one another, but I'd be interested to see because I think in terms of musical and thematic structure both The Whirlwind and SDOIT are very similar. Both have overtures and regularly refrained melodies and, thematically, both deal with how something (hardship-God/emotional disorders) affect people without really telling much of an overarching story.