There's no reason to avoid answering that question.
At one time, in the USA, I was a contingency-based recruiter. If you are already in interviews with that company, through your recruiter, then any position they hire you for is going to generate a payment to the recruiter. At the time you are submitted, and accepted as a candidate, they are agreeing that it was the recruiter who brought you to the attention of the company, and that they were unaware of you, as a candidate, before that. This is usually good for any contact with you over the next six months to a year.
In this case, it seems like they are all internal recruiters. This one probably liked the interaction they had with you. Maybe they looked at your LinkedIn profile and saw that you might be qualified for positions they are in charge of filling. And, yet, they did not already know of you, personally, even though others in the company obviously do, if you are interviewing.
Since you are already interviewing, that other recruiter has already done much of the preliminary work, and they probably can't initiate a process on their systems with a duplicate candidate. The recruiter you exchanged messages with probably needs to make sure they aren't chasing someone who will already be hired for another position, or perhaps aren't pursuing someone who got red-flagged in that other interview process.
In any case, the most logical reason for asking is because they probably have their own positions they might think you are a fit for.