well I mean how much faith can you put into a figure that is by all means, calculated or "estimated" from a given figure (wheel horsepower)?? On a rolling road Dynamometer the "Drivetrain" loss is calculated by the remaining force of the wheels turning the drums when the car is stuck in neutral after the power run...Now think of the inconsistencies - you have internal friction resistance of the drum, you're going to have friction loss between the tyres and the drum as you cannot guarantee no slip whatsoever, and so on...
I mean, not to discredit the guy at your Dyno, but I very much doubt he would say something that would criticise the very device on which the business is based on. In my opinion, Flywheel Horsepower can only ACCURATELY be calculated when the engine is bolted to an engine dynamometer, as the power is being taken from the flywheel. In that respect, using a Rolling Road or Hub connected (much more accurate as it does away with insoncistencies of traction and mimimal losses through the flexing of the tyres etc...) Dynamometer, will give a more accurate reading of the power at the wheels as in effect, the wheels are driving the Dynamometer, not the flywheel.
It is also relative though...the calculated drivetrain loss would be the same level (well, depending on the car's ACTUAL drivetrain loss) for any car on the same dyno, so the inaccuracy of the flywheel horsepower will be the same for that particular dyno regardless of car.
There's lots of methods to calculate drivetrain loss...but each sort of continent has different "estimates" for Drivetrain loss - Australia for example claim about a 15% loss, USA about 20%, UK something silly like 30% and so on...I mean, I cannot say for sure, I'm just using my own theory to guesstimate the above, and I think it makes sense. Plus if you research, you will find the answers to this as well. It is why the States and Australasia SOLELY quote power figures, as WHP. Only manufacturers really have the right to quote Flywheel Horsepower, as they'll have it strapped to an Engine Dyno