Dirt bikes like Enduros and MX are stand up bikes too. That is how they are meant to be ridden. They do allow for sitting down because they are ridden longer distances than trials bikes and not all of that distance requires standing, whereas a Trials bike you are standing all the time in a competition.
The reason their seat height is so high is because of rear wheel travel. If you remove the seat you only get a few less inches of height.
I have no illusions about what an Alp is - I know it is a "mostly" standup bike and even if it wasn't it probably would not be ergonomic for me to do so. It won't be as bad as a Trials bike though.
Mostly I would want a bike like that for practicing my technique on something that is much lighter and is a bit easier for me to learn and practice on - especially at slower speeds. I've been riding for decades, and I started out off-road, but I am a very poor rider so I really need to practice, get in shape and develop some skill and endurance.
If I get such a bike, I would have to have it re-sprung though - I am way over the target weight for the bike, and as I learned with my Hussy, the proper suspension setup makes a bike difference in how the bike handles.