Will my methods chapter be less solid, if it is based on one book or the work of one scholar? I'm writing a Bachelor research paper using netnography or virtual ethnography.

I've found that this new methodology was mostly advanced by two scholars: Kozinets and Christine Hine. 

I managed to get Kozinets' book (2010) published by Sage, in addition to some of his papers, which I think is good in terms authority and currency.
I also have authoritative sources by Christine Hine but are more theoretical and do not offer step-by-step guidance like that of Kozinets.

Will my methods chapter be less solid, if it is based on one book or the work of one scholar?

Thank you
Literature on virtual ethnography or netnography is not restricted only to these authors. A cursory Google Scholar search reveals many other authors. 

Having said this, ~50% of my methodological approach in my work is through grounded theory by using semi-structured interviews. The gold standard for grounded theory is:

Glaser, Barney G &amp; Strauss, Anselm L., 1967. The Discovery of Grounded Theory: Strategies for Qualitative Research, Chicago, Aldine Publishing Company


... and I would wager that ~95% of papers using grounded theory cite only them in the methods sections of their papers.

This does not necessarily make your methods poorer or stronger. You should make sure that you understand the methodological process perfectly and then implement it. 

Make sure that you do talk about credibility, transferability, dependability and confirmability as well !