The long hole is to give the drill clearance and allow for several sizes. It workes for me from drills 3mm to 8mm. These are the ones I use most of the time.
The other hole is dimensioned with 3.3mm - core hole for M4 thread. You have to do the threading by hand. With PLA that worked great for me. I currently print the part with PETG and want to try that with that kind of material also.
I currently print the part with PETG. Update follows, if it works as good as PLA.
Printing itself with PETG was good and the final looked the same as the one made from PLA. But I could not drill the M4 thread into the core hole. After a few turnes the material broke and I had to throw away the part.