Putting ISO aside, you end up believing that ISO is the only difference?
Try a roll of tungsten balanced and a roll of daylight balanced film.
Compare how both perform, with appropriate filter, on each other's turf.
ISO is not the only difference.
There may be a difference in practice, but there is no need for there being one. This is not magic. Tungsten light differs from day light in the distribution of power density over wave length, and a suitable filter should be able to even this out. Chances are that 80A doesn't compensate well, and looking at the transmission curves from the heliopan filters I posted before, this is certainly far away from a perfect compensation filter for any incandescent light spectrum. The next question is obviously "what is the spectrum of day light anyway?", since this in turn depends on daytime, season, weather, altitude and latitude.
So what's my point? It is hard to believe for me that any film can yield perfect colors in any standard setup, unless the setup is deliberately matched to the film type. We may (and most of the time do) accept the look of our resulting slides as good or adequate, but few to none of us know the exact spectral composition of all light sources involved, the exact spectral response of the film emulsion (ideally the batch lot we're using), and the spectral transmission curves of all filters and the lens. With carefully selected filters one has at least a chance to get accurate colors (up to the limits of the film used). This may turn out to be a lot more complex than just slapping an 80A in front of my lens, and tungsten balanced film may indeed accomplish the task much easier, I just take your word for it.
BTW I just ran a roll of 64T with halogen light and the results after scanning look comparable to Astia 100 in day light. I hope I'll get that pack of Provia 400X soon, so I can try the same with an 80A filter in front. I will report about the results.