I remember seeing and even asking the same on here as a FTM, I honestly seen it so often I thought there was some exception I didn't know about. I will never quite understand that. However it seems like an uneducated thing here rather than ignorance. I don't know what I'd do in your shoes, normally I feel that it's not my place to say but it sounds like she doesn't have a lot of guidance.
My lo was 5 weeks prem and I delayed the weaning process till her adjusted age met the right weaning age. Has a dietician not spoken to them about how preemie babies should do things at their adjusted age and not actual age???
The advice here is actual age for premature infants (5-7months). There is information on the Bliss website, and our neonatalogists agree with this. Worth looking into.
There's an idea OP!! Send her the link to the Bliss weaning guidelines, say you saw it and with her being a preemie it might be helpful! https://www.bliss.org.uk/help-for-families/feeding/weaning/ you could send it with a few other links on there, they are of great tips and support