Orks Mob Rule in Warhammer 40k
I'm afraid you do understand the ruling correctly (there is no GW statement on this that I'm aware of since there's no conflict in the two rules). The important thing to remember is that the substitution is optional - if you're likely to take massive damage from fearless, take a chance on the dice and use their adjusted leadership (adjusted for modifiers from the combat). I think that you will find that your decision on taking casualties vs risking a failed leadership check will greatly depend on the unit and what you're trying to do with it (a unit of 30 gretchin designed to hold up an assault squad is going to happily take the casualties while your boyz are likely to take their chances with a roll - though even these examples are dependent on what you're trying to do and the current situation on the table).