(Under the implicit assumption that the supervisor is a (full) professor or some other similarly high-level person)
PhD supervisors typically already have a lot of duties: teaching classes, marking exams, conducting oral exams, supervise the projects of the other PhD and grad/undergrad students, proofread papers, theses and reports, grade theses and reporst, manage administrative duties, prepare presentations for conferences/visits to other institutes, be a host to guests from other institutes, attend department meetings, and many, many more.
At the same time, the group has experienced PhD students and postdocs that are right on the cutting edge of the research but who have much more time for side activities because they only have a small fraction of the rat’s tail of duties the (full) professor has.
Having the full professor, i.e. the official supervisor, be the person teaching the new PhD student would be very inefficient. It is, by definition, a one on one situation meaning a high time cost with a necessarily low gain. Something that really doesn’t fit well with tight schedules. On the other hand, experienced PhD students and postdocs also have the knowledge and much more time on their hands. Therefore, if there is any actual teaching of methods or skills to be done, it will be the PhD students’ or postdocs’ job.
Naturally, the supervisor will still be there for the core part of their job: answer questions, provide guidance and point to helpful references if they are hard to find. I believe the quote states your friend’s supervisor is planning on doing exactly that.