So what if she isn't, you can just get her to have an affair with you, problem solved.
Comfort the disturbed and disturb the comfortable

If it's a rumour on the net, then it must be true.
But seriously, I have no idea, and (to me) it doesn't really matter. I just hope she's happy.
But why do you doubt she'd get engaged that quick?
Plenty of people do that, it doesn't mean marriage is going to follow soon after. Some people stay engaged for a long while without taking the next step.
Plus, you don't her and what she's like, or how she feels or thinks. Meaning this in a nice way, it sounds like you're projecting your ideas of what you think she's like onto her. She may well be impulsive, she may well feel that she's completely in love and this is the one for her. She may have known the guy for a lot longer as a friend before they dated, making the 5 months irrelvant.
Unless she says anything herself then we have no way of knowing. It's all just speculation.