Steven liked to finger parallel scales so that the thumbs play together. The big problems in scales are to get them smooth, without bumps, and to keep the hands perfectly together. The turn over/turn under in scale playing is hard. If you align the fingering to play both thumbs together, it gives you a little more of a fighting chance to smooth that out.
In the traditional fingering of a B-flat major scale there is no meeting point of the thumbs. However, you can make the thumbs meet if you wish to. In a parallel B-flat major scale, Steven would put his left thumb on C and F.
C major would have the left thumb on C and F, D major on D and G, etc. This isn't dogma. It's just a choice if you have a passage that has a long parallel scale or even a fragment that could benefit from getting the thumbs together.
It's not traditional. I don't teach children, and I don't know when it would be appropriate to introduce them to such an idea. I learned that in college.
It's another choice. Maybe someone else will like it, too.