While there are oversized rear axles available, those go on bikes that are maybe 10 times more expensive than the one you linked to, if not more.
Most rear wheels share the same axle diameter.
But the key isn't in the diameter, it's in how much the sprocket stack overhang the bearing.
With a freewheel design, the inside face of the bearing sits pretty much flush with the spoke flange. The rest of the distance to clear the sprocket stack and to fetch up against the inside face of the dropout is taken up by spacers.
With a freehub/cassette design, the bearing sits something like 20 mm further out. Meaning it will be that much closer to the dropout.Meanin it'll need less spacers. Meaning it'll be a lot less bending force to deal with. Meaning the axle will survive better.
But you're right so far that it's the same principal design. The devil is in the details.