[Scythels]

 

The other way around may be true, that how they caused the background bokeh to be smooth in s simple way caused that the in focus plane is not very sharp. But only real lens designers who can do measurements on the different lens elements of this lens can tell if that is the case.

Optics isn't magic, there are no cause-effect relations that work one way and not the other.  The 58/1.4 has immense spherical aberration.  This gives it a very hazy appearance (=soft focal plane) but also causes things to be thrown further out of focus than they "should be" in a perfect lens.  This greater distance leads to a greater blur radius, and thus softer bokeh...