hello
@Antkin
and welcome to TP
it's that first time a question or comment comes up that floors us, isn't it
I suspect that now you've had time to ponder, you've probably thought of a few things you might say should your mum mention this again
personally I'd tend to go along the reassurance route also - and if it will keep your mum settled rather than anxious, use some 'love lies'
maybe 'well we're all getting older mum, but there's a some good times left in us both I think so let's enjoy ourselves, how about a ... (whatever she considers a treat eg I keep chocolate and chocolate biscuits close by at all times for dad and take him in his favourite fruit each visit)
I agree that actually discussing dementia may not be productive or comforting, even if your mum has generally been the type to want to know things - and often the diagnosis isn't retained by the person anyway - so any 'wobbles' with dad I tell him are part of this getting older business, or because he's tired, may be starting a cold ..... so I sympathise and reassure and then distract/change the subject if at all possible