This is one of a collection of rulings that seem to contradict each other.
The Legendary Fisherman says it cannot be targetted, so your opponent can target your life points directly as long as there isn't another monster on your side of the field. This despite the fact that the manual says you cannot do damage to the opponent's life points if there is a monster on the opponents side of the field (unless a card specifically overrides it, and this card does not specifically override that, it just overrides the ability to select a certain monster).
This pretty much states that because he cant be selected as a target for attacks, he essentially becomes invisible to the field if he is the controllers only monster, allowing for a direct attack.
And then there's
Command Knight, whose text is similar (using the words "cannot select") but add a condition of another monster on the field. So if there's two
Command Knights, you cannot target either. Yet, this is somehow different from TLF, because you also cannot target Life Points (unless you have a card effect that allows you to). And it is also somehow different than
Solar Flare Dragon.
Command Knight follows the "You cant attack me If there is another monster, so you have to attack him" attitude, and because you have 2 of them together, the other one says "Hey you can't attack me either!" Because of the fact that Command Knight must have another monster present to exclude him from being a target, that essentially prevents a direct attack on life points, even if that other monster is "Command Knight" by bouncing you back in forth between the two of them.
Solar Flare Dragon says it cannot be attacked (nothing about targetting) if there is another pyro type monster on the field. And if there are two
Solar Flare Dragons, then the opponent cannot attack but can still declare battle and can still, apparently, target but cannot attack) Does it help to target but not attack? I'm not sure, I couldn't think of any cards that let you destroy the monster without going through damage calculation.
Solar Flare Dragon basically does the same thing as Command Knight. "Dont attack me, attack him!"
And then there's
Marauding Captain. Your opponent can ONLY target him, unless there are two of them on your side of the field, and then suddenly you can't target either one of them. But somehow the
Ring of Magnetism overrides this, even though the ring forces attacks to the MC. Somehow you were able to target the other MC, even though supposedly you can't, and the ring redirected the attack to the equipped MC. That just doesn't make sense.
Marauding Captain is different from the above group as he designates a type monster that can't be attacked (Warriors), with the main exception being he is not protected by his own effect. "Hey, you cant attack him cause he's a Warrior! Attack me!" Having another one on the field means that the 2nd one says, "Wait, He's a Warrior also, so I'll protect him as well!" With both of them saying "Don't attack him, attack me", your opponent cannot choose to attack directly since you must attack either monster if you would attack, but you are prevented from doing so.
The lock is not really a lock "from attack", its actually a lock "to attack", but you are "locked into" attacking a monster that can't be attacked.
The group of rulings is inconsistent. But who cares: "Konami Says So".