Maybe. Maybe not. I'm not saying go elliptical instead of go for a ride. The best cyclists in the world are going to be the best whether they ride a TT bike or a road bike. If someone prefers training on one over the other, it's not going to make them incapable of riding fast on the other.

The original comment I was replying to was claiming someone must "must not do long distance triathlons very quickly then" and "because that is the best way to develop positional fitness required to be competitive"

which is straight up incorrect. You can be very quick and competitive even if you do the bulk of your training on a road bike. Will you be winning Kona? Maybe not. But there's no reason why you can't compete well and continue to improve training on two different bikes.

Also, everyone also does tris for different reasons. For me, I got burnt out of running. I ran a 2:33 marathon in 2013 and haven't gotten close since. I've accepted I'm never going to PR in the marathon again. The best thing about cycling for me is being able to see new places that I'd never see on my runs. If I wanted to be specific, I'd just ride the same boring flat ride over and over at fast speeds to practice racing. But that would ruin it for me. I passed plenty of people with $4000+ tri bikes this weekend on my lowly aluminum tri bike. Would I have passed a few more had I exclusively trained on my Tri bike? Maybe. But I'd also probably not be as motivated to get out for a ride if I wasn't going exploring mountain roads.