Thanks for your response!
To take the makephilosophy account as an example: Whenever I click on their username on their jam submission page, I'm taken to a URL with the following format: https://itch.io/profile/makephilosophy
Even when I manually input a URL in the other format (i.e., makephilosophy.itch.io), I'm automatically redirected to the first URL. When I try to input either of them on the Contributors & Judges panel for my jam, I get an error that claims this account doesn't exist.
I hope this additional information is helpful! And thanks again for your time.