Well that really depends on her because your conclusion is honestly not the kind of conclusion that I would've came up with. Some people are comfortable talking about their exes only to certain types of people (people who they deem as close usually) and there are some who don't mind sharing their story to basically the entire world (and I am one of those people). Also some people are not willing to talk about their ex at all if the breakup was fresh and would only talk about them freely once their feelings have subsided. That's why having someone talk about their ex to you may or may not be a significant thing for them so it's not a good indication of their feelings towards you.
Although I must admit that when someone talks about their ex (especially if its excessive), it usually means that they are somehow still thinking about them (unless they're the kind who don't mind sharing....then that's a bit tricky to judge). It could be that they still have feelings for them, that their past relationship is still haunting them, etc. Whatever the reason is, if she appears to be still hung up over her ex for some reason, she probably isn't ready to move on yet.
But from what you said, this sounds more like she just briefly mentioned her ex rather than spilling her whole relationship with him and excessively talking about him. Mentioning an ex is different than really talking about them. Mentioning an ex is by far easier to do and can sometimes be less intentional/meaningful than having a heart-to-heart conversation about their previous relationship. In other words, her mentioning her ex may just be her mentioning her ex and nothing more.
Okay that aside, the two of you have at least found some common ground to talk about which is this a7x (yeah I don't know what it is but the two of you clearly do and that's what matters!). So instead of reading all the tiny things that she might be unintentionally doing (such as the whole ex thing), I would try focusing on what you can do to up your game. I know that it's hard to not read into every little thing but trust me, it's not worth it because they are often unintentional and thus unreliable. That's why if I were you, I'd continue talking to this girl about this a7x. But if she finds that it reminds her too much of her ex, maybe you can suggest something that is similar but new that she could enjoy. I don't know, be creative and improvise when the situation arises. Good luck!