As far as I know, there's no (direct, retrograde) transfer of pathogens from the child to the mother through the breast. But if the child has an infection, there is often a higher chance for the mother to get ill as well (due to close body contact, aerosol particles, etc.) and to develop a "mature immune response", which could lead to a (delayed) positive secondary effect on the child through passive immunity.
And the link from that post is now broken... so the author took down the previous time they posted this to their substack, and then reposted it with today's date?
As a side note, the German associations of oncology publish their guidelines here (HTML and SVG graphs): https://www.onkopedia.com/de/onkopedia/guidelines