I think there’s a “difference” for TT or tri. The assumption of the 1.5x outside time on an indoor trainer includes the breaks, coasting, stop signs, etc. that you hit during most road group rides, any pack riding/racing, or even solo rides on rolling courses. If you’re doing a flat TT or triathlon, you’re applying force to the pedals for far more of the interval, with the only exceptions being turnarounds for the most part. I’d have to go back and look at my file of my last 70.3, but I would guess I was pedaling for at least 2:25 of the 2:29 bike leg, if not more.
So I think you’re right… if you’re training for a 40K TT or Oly (or shorter) bike leg, you probably need to be able to apply the appropriate force to the pedals for the appropriate amount of time. It probably lessens in importance as the TT/bike leg distance goes up because the required IF goes down (e.g. if you can ride 2 hours at .83 IF when you’re in the middle of your build, you’re probably going to be fine riding 2:30 at 0.83 IF after a taper and still have good legs for a run).