The quotes did not come from a published work, they were communicated directly to the author.
Because readers cannot retrieve the information in personal communications, personal communications are not included in the reference list; they are cited in the text only. https://apastyle.apa.org/style-grammar-guidelines/citations/personal-communications
As suggested in the above article, the formal way of doing this is to write "personal communication" in full, and include the date it was received.
The author has used a less formal approach by abbreviating and not giving the date. However it is clear from the context that this is the meaning because the quote says, "... as pointed out to me by Staffan Hellberg ..." and that is clearly a personal communication.