Any wheel with slopes can exhibit upward smash. Upward smash is smashing the opponent with an angled contact point, so it perfectly fits those requirements.
That said, from your description, yes, it is possible that that was upper attack, as upward smash will normally push an opponent away, not up. That said, as you can see it wasn't particularly effective.
As for what happened with Inferno Byxis, that sounds more like upward smash, but yeah. It's not impossible at all for a wheel to do both.
I guess it could be between the two, it's kind of a spectrum on which we mark major points based on the relative time of the lift and the "hit".
Upper Attack, the lift happens first, followed by the hit.
The weird plastic combo, the lift happens first, but the smash is there, however, the lift has the first effect, and the "smash" effect takes place after the lift effect, so in effect, it is upper attack, as there is first the lift and then once lifted, the smash.
Upward Smash: the lift is caused by the hit, it hits in an upward-angled direction. Basically, it's smash in a slightly different direction.
Smash is just a hit.
Let's be honest here though, I'm not great at physics and I haven't seen what's going on with your beys myself so I'm rather limited here.
EDIT: Okay I managed to dig up my Beylauncher (though it's string is shortened and it isn't smooth because there's a knot of frayed string), and give a shot at upper attack/upward smash in similar settings. Firstly, Metal System Bull is, as usual, better at everything than midnight, but anyway. I did manage to see some upper attack, yes. Very few KO's from it, but I did see what you described. Most of the KO's came from upward smash, as far as I could tell (as I said, upward smash and upper attack look similar, because overall they have a similar effect, but Upper Attack looks more like a "bounce out" than a "knock out".) So yes, both M.S. Bull and Midnight can exhibit some amount of upper attack, but it's far less significant than their upward smash, because the contact time and so on isn't there, and they don't really have any "smash" to follow it up with to make it effective. I think the latter is the main restriction, the only smash comes from the slopes themselves, and if it's powerful enough you generally see upward smash instead, as it hits hard enough to KO before lifting, whereas otherwise it just doesn't hit hard enough to KO, and just lifts the opponent (which is basically destabilisation).
I need to revise my upper attack/upward smash thing, but yeah, the lines do blur, but upper attack just isn't an efficient tactic in MFB for a few reasons, the design of attack wheels (lacking the smash to follow up), the short contact time, the lack of significant control of weight distribution, and the control of height, which makes it much harder for upper attackers to get under anything/lift opponents properly.