Leo wasn't just a Reader, he was also someone who traversed between worlds!
Getting stamped by some obscure entity wasn't an option. Not only could it remain on him when he went back, but there was also a possibility that this could prevent him from returning, to begin with.
Most importantly, there was no way Leo would allow himself to become someone's slave, especially not for the sake of power.
'I will deservedly make it my own!'
Although the monstrosity couldn't be tricked to take out the Enigma Stone by itself before it imprinted the receiver, the Enigma Stone itself had no requirements.
One didn't have to possess the imprint to use it. It was just like any other Enigma Stone that enriched its user with something new.
The only difference was that it was a special one. Even the strongest monster on Earth couldn't produce something with a similar effect, and that was all because the monstrosity was made that way.
