But there are screws on the side for mounting a 3.5 in hard drive and it said d it could be used for a 3.5 in desktop hard drive.
No, that's not what it says at all. It says it converts a 2.5 inch hard drive into the
form factor of the 3.5 in desktop hard drive. It repeatedly tells you to use 2.5" drives.
A 3.5" hard drive doesn't need a caddy to go in a 3.5" drive bay because the 3.5" drive bay of a PC case is already
designed with its own solution to natively use 3.5" hard drives.
It not only says "for use with up to two 2.5" SSDs" and "supports 2.5 hard drives" it literally has a picture of how it's converting a 2.5" drive into the same form factor as a 3.5" drive.
The screws on the side are to mount the 2.5" drive in the bracket to a case in the normal way you would a 3.5" drive. The holes on the side aren't there for a 3.5" hard drive, they're there to connect the
bracket to the case. And since the bracket is designed to work with cases, the holes to attach the bracket to the case are naturally going to be in the same place as where the holes are in 3.5" drives.