It's basically true. I am this way. I'm only shy around guys I like, but not around people in general and it can be really hard to hold eye contact with a guy you like. I usually avoid his eyes because it makes me too nervous and uncomfortable. When I'm close to the guy I like I get too nervous to look him in the eye, but when I am on the other side of the room I generally try to hold eye contact as much as possible which is usually 2-4 seconds. Also, if I look the guy I like in the eyes, I blush and it totally gives me away and sometimes if we are not sure that the guy likes us back, we don't want them to know that we like them because of fear of rejection.
The thing is that girls don't want to look stupid and if there is a chance that the guy we like isn't interested, it would make us feel stupid to hold eye contact. Once we are basically sure that the guy we like may like us back, we start holding eye contact a little longer. If the guy initiates the eye contact, I get more comfortable and reassured that he may actually like me and so I will hold it back.
The thing about shy girls like this is that if she likes you, she will make some sort of an effort to show you. She may TRY to hold eye contact or ATTEMPT to flirt with you. If she is making some sort of attempt, failed or not at trying to show you she's interested, she most likely likes you. If she is not even trying to hold eye contact or anything, she may not be interested or just be really really shy. I am shy around guys I like, but I always make some sort of attempt to let him know I'm interested.