Most of the times, cases like yours can be attributed to actual wrong credentials despite the fact that one thinks they are correct.
The WordPress login page doesn’t have any relations to the client area nor the control panel. If you used the same password as dash.infinityfree.com or cpanel.infinityfree.com (or FTP) then it is the wrong password.
Your case looks like that you just let Softaculous generate their password so you don’t know what the password is and can only access WordPress through Softaculous.
In this case, you must login to FTP to delete unwanted files to be able to access Softaculous again.
The title of the article is a bit misleading, as the process is very similar for deleting files.