But ProPhoto RGB is not an input profile.
ProPhotoRGB is a matrix based profile with defined primaries (TRC, WP ...).
The "LeafLF3-ProPhoto RGB.icm" (same as the "LeafLF2-ProPhoto RGB.icm") is a table based input profile. It also shows different primaries.
Of course I don't know in detail but basically it goes like this...:
At the first stage the RAW files in C1 get transformed from non-linear data into linear data and get converted into C1's internal color space (which is huge).
This pre-transformation takes place under the hood and is basically the same as in any other RAW software.
In the user interface there is a tool where you can select a "camera profile". These user-selectable profiles are assigned to the (pre-transformed) RAW files and they do apply a look. They also provide a neutral gray axis (mostly Gamma 1.8 ). So these "Camera Profiles" in Capture One are designed as "camera specific working spaces"... if you want to say so.
The "LeafLF3-ProPhoto RGB.icm" profile is designed to assign a very neutral and pretty flat look to the files (actually it applies "no look" and leaves the creation of a specific look to the end user). As a "camera profile" in C1 it provides an optimal starting point for further editing in ProPhoto-RGB (which you can choose as target when processing). That's all.
So since the "Leaf-ProPhoto.icm" is NOT ProPhoto-RGB it is very good not to call it "ProPhoto".
Since it's desigend as a specific starting point for further editing in ProPhoto it is usefull to call it "Leaf ProPhoto".