Right, that does pose an additional issue for the non-sentient hypothetical being. However, it doesn't really break the argument. We can reduce it for example to people who are born without certain senses - for example no eyesight. These people just can't know what it is like to see (even though some scientific evidence shows the visual cortex does engage in some ways for these people) - so in general we know that without being acquainted with what-its-likeness.. you simply can't know of it.