I don't think they have the means to pierce through the shells, from what I have witnessed in my tanks and from what I gather from any information online is that the assassins will "pounce", for the lack of a better term

, onto oncoming snails and devour them through that way and do not bother with those with a strong operculum.
This is what I have witnessed in not only my own tank but several fellow hobbyist as well. The "piercing" method you speak of is very interesting though and I would love to see pictures of the aftermath!