welcome to DBs, Ulc!
You are absolutely correct, if the mutation algorithm worked by deleting and/or inserting single letters, then the chances of getting any meaningful mutation would be very low. It's not to say that the program would not work, it's just that simulations would take very long time. So we just assume that any mutation that results in a defunct programming code is lethal and we don't even bother to create a bot with it. Well, actually we don't bother even to create a mutation like that. It is really just a shortcut, but it works very well!