How do you know it's a one piece wheel? I didn't see any info on their website that describes whether it's one or two piece. You can't tell by looking at the pic.
Call them and ask them how do they adjust the back space to your specs? Is the wheel cast with that back space? I doubt it.
Are the wheel center and rim made separately and then welded together at the backspace specified by customer? If so, that is a two piece wheel because two pieces (rim and spokes/center) are welded together.
I wouldn't trust a welded wheel.
Call them and ask those questions.