Perhaps the people in your group know that it would be best (on paper) for their child to have no dummy, but there are other reasons their child has come to rely on it, for example sensory needs which aren't yet diagnosed, or bad reflux meaning sleep was a problem, the family's work schedule meaning toughing out the sleepless nights is too much to manage right now, or maybe just the anxious personality of the child.
There is a really hard thing in parenting, when you look at what someone recommends as the actual "best" for your child - and then have to come to terms with not being able to ensure that happens, for other reasons, such as the distress of lack of consent of the child, or other practical reasons.
As a SEND parent I have had to learn that really brutally about the biggest things - food, medication, sleep, dental care, education. People who get divorced might have to learn that too about some things, e.g. not being able to ensure a toddler can have the basic stability of both parents every day, and doing their best under those circumstances.
And of course people with even worse situations, people in wars or in ICE detention or what have you, who cannot give their children what they know is right for them to have. The pain must be excruciating.
Most people don't have to live with that level of difficulty of course! However I am sure everyone at some point feels the pain of thinking "the optimum care for this child is X, but I am only able to do Y".
Maybe the mums in your group are simply trying to handle this. They know "losing the dummy is best" but in practice it isn't as simple. Talking about it is a way of processing some shame and grief around that.
OK so a dummy is a smallish thing but this could be the first time they are realising that parenting isn't just "follow these instructions to the letter" but a range of competing demands in which you often do not succeed in creating the perfect outcome.
I think you have a bit of privilege going on which is great for you OP - i wouldn't wish the above realisation on anyone as it isn't fun - but you should be aware that it's a challenge you have not yet met, not just a failing on their part.