sqlplus user/password as sysdba uses OS authentication by default, and bypasses the username and password check, so as you said, you can provide any password or username.
Logging in from SQL Developer requires a remote session. For a remote SYSDBA connection, you need the following:
SHARED, but not
- create a password file for the database if it doesn't already exist:
$ orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID
After this you should be able to log in with SYS and the password you provided.