For RAW you're absolutely right. And they didn't say anything about it in the FAQ, so I suppose they're happy with the RAW that lets Harlies provide cover saves. Makes sense -- you have a scintilating psychic veil of illuions between you and your target, it's gonna throw off your aim a little...
Then again, RAI might turn out differently. Back in 3rd ed, when there was still meatshielding, they made the rules absolutely clear that Stealthsuits could not block LOS. Of course, I think those rules are gone because Tau Empire was released during 4th ed when meatshielding didn't exist. At this point, it's just guessing how they want it to turn out. Certainly, at one point they were opposed to invisible guys being used as screens, but there's a new team in charge of 40k, and many years have passed, so they may be cool with it now. Especially since the FAQ is silent on the subject.