Hey!
I would say that this is absolutely fine, and it can be a really nice way to show genuine insight if you frame it well!
I did something very similar in my Reed Smith interview. I'd had a coffee chat with a partner during one of my summer vacation schemes who mentioned that they had trained at Reed Smith and spoke really highly of the culture and their experience there before moving firms. I then referenced that conversation in my final partner interview during the AC as a way of showing that I had gained insights into the firm's culture, and explained why I was drawn to it (especially as a direct TC applicant, where I didn't have the VS experience at Reed Smith to draw on in the same way).
It actually worked really well - funnily enough, the partners interviewing me had worked with that partner previously, so it led to a really natural and engaging conversation about the work in that team.
I completely get the concern about whether it might come across the wrong way or feel slightly indirect (as this is what I worried about too), but I think the key part is how you frame it. As long as you are clearly linking it back to your motivation for that specific firm (e.g. what you learned about the culture, training or work), it adds value rather than seeming irrelevant. I found it was a really good way to show that I had (indirectly!) tried to learn more about the firm and went beyond surface level research
