Other mistake: In the beginning of the film when you're introduced to the younger, humanoid Sil in the laboratory, an extreme close-up of her eyes and face clearly reveals the young actor's contact lenses.
Other mistake: When Sil jumps through the hotel room wall, Pres fires two shots from his Glock and then gets in the elevator. In the elevator he drops the magazine and his Glock has the slide locked back showing the magazine is empty. The man is an acknowledged expert in special ops (Indeed, the fact that he changes magazines in the elevator demonstrates his preparedness. He wants to face Sil with every bullet he can, he doesn't want to be even two short.) He is going up against an extremely dangerous extraterrestrial life form that has already killed several people. There is absolutely no way he would be carrying a weapon that was not fully loaded.
Other mistake: At the end, during the final showdown, Laura falls into the oil and it is deep enough for her to need to swim in it, but she is completely clean afterwards. Even her jeans are spotless in the next scene when they reenter the sewers and the camera is zoomed out, showing her legs.
Answer: You could probably make a compelling argument that, as an alien-hybrid, Sil is stronger than a normal person and thus was just able to forcibly cut through the finger with the pruners by "crushing" it. Also, this is where good old suspension of disbelief comes in. The pruners might realistically have broken at some point, but it's a movie, so we can "forgive" this bit of unrealism because it creates a compelling scene.
TedStixon