Need to know if the advice I got was correct regarding a new bike

I went to the bike store yesterday to take a look at getting a bike computer because I want to start tracking my gains or losses accurately for training. While I was there I thought I would tap the owner of the shop on some advice on getting a tri specific bike and what size I need and such. He was very helpful. Seemed to be steering me towards a Cannondale. I asked why not Trek as that is what I am on now. He said the rims they come with are not as strong for my weight, I am currently 250 and 6'4". He explain that the widely spaced twin spoke design is not as strong as an and equally space spoked design, and that the twin spoke design is for Aerodynamics but for the Aerodynamic effects you loose strength. All sounds plausible. No problems here. He went to get me a catalog and wrote down some prices in the catalog for Cannondale. He then asked " what are you riding now" I said a Trek Mountain bike, no suspension, with road tires. He then seemed kind of taken back. Said that getting a Tri bike is not the course I want to go. That even pro triatheltes don't train on tri bikes. They only use those for the races because the position you sit in with the aero bars is uncomfortable. He said I should just get a road bike and then I could switch components as I go along to transition to a tri bike, then eventually get a tri bike

This doesn't make sense to me, how could you not train on the very instrument you are going to use in your race? Sort of like don't train on a treadmill because it does not train you accurately for what you will encounter in a race? I can't imagine training in a totally different riding position all the time and the suddenly on race day plop onto a tri bike and be ready to sit like that for how ever long your race is. Does this sound right????