I do see what you mean Jemima - but equally, if you said to someone who didn't have DCs when you meet up, they can't talk about their work life, their studies, their career goals etc, or their hobbies, any recent health issues or in fact not talk about anything they'd done in the last 2-3 weeks or anything they are planning to do for the next month or so, then it would cause most people problems in carrying on conversation!
Once you've had DCs, particularly in the first few months when you are on mat leave, your child isn't like any other relation, they are your family, your job, your new love, are involved with all your 'free time'.
It's not like saying "don't talk about your boyfriend" or "dn't talk about your wedding" or anything else going on in someone's life before DCs, because very few people had something going on that absorbed so much of their time and attention, jobs, relationships, house moves etc, generally have other stuff going on too. New baby isn't in the same league of 'attention'.
While banging on about your DCs all the time might be dull, banning talking about them effectively means "don't talk about your life". Once DCs are more independent, you can have much more of a separate life from your children, but at the first few months stage, there's little going on that's not in someway related to them.
A friend where you feel you can't talk about your life, only listen to them talk about theirs, isn't a friend that's really worth keeping. It might be possible to just stick them on the back burner for 3-4 years until you have more 'non-child life', but if you are mentally saying "I'm not going to be able to have a good chat and fun with this old uni friend for longer than we were at uni in the first place", is there much point waiting?