I was in the same boat as you. Never ridden a motorcycle, but very comfortable on my mountain bike.
Turns out they are not even in the same ballpark.
Have you ever driven a manual car? That would help far more than riding a mountain bike.
My recommendation is to do the course with their bikes. It'll likely be a small 125cc, but it's perfect for learning on. The difference between what you'll learn on, and what you buy really isn't much. You'll come away with either wanting to continue on a bike, or not. Do this before spending thousands on a motorcycle. It's a no-brainer.
If you like it, go ahead and buy yourself a bike. Like I said, it's gonna feel very similar to the bike you learned on unless you buy a very heavy bike (don't do this to start).
The smart decision is to buy secondhand for your first bike, so if (when) you drop it, it's not a big deal. You also don't want anything too powerful. I got a Yamaha R3, and it's fast enough to get started on. Certainly faster then most vehicles on the road.
In summary, do the course first. A 400 is more then enough for your first bike, and your mountain bike experience doesn't really count for anything unfortunately.
I don't think it is irrelevant though. in a car that can and does control your throttle without your input, and is famously poorly built with tons of software issues, built by a company whose head is a famous conman, you can't assume that this is driver error, even if it is a semi-common driver error.