On a technical level, I have heard second hand what your dealer and TomS above are saying. Bennington told me something similar on not needing to support pontoons with lift or Sea Legs. I spoke to someone in 2019 as we switched from tritoon cradle bunk lift & canopy system (supported) to Sea Legs (unsupported).
In my mind I see two issues with your trailer situation: a) support concerns over time and b) the price for what you got.
A) If going a very short distance on a smooth road to just drop my boat in and pull it out annually, I wouldn’t worry about it from an engineering/support perspective. However, if towing all the time to use it, and/or towing any sort of distances, I would 100% have a 3 bunk tritoon trailer. I would never feel comfortable towing distances or regularly unless all three bunks were supported.
B) Finally, at the price you paid, I would also 100% expect a 3 bunk tritoon trailer. Otherwise, it absolutely seems like they are screwing you in that you are paying for a very nice tritoon trailer in price, but not receiving it. Like you mention, and BigD echos, you can save a couple thousand if just buying a bi-toon trailer. So also regardless of engineering specs, IMO you are overpaying for what they delivered to you in a trailer.