Could be following British convention.
https://www.thepunctuationguide.com/british-versus-american-style.html
American style uses double quotes (“) for initial quotations, then single quotes (‘) for quotations within the initial quotation.
British style uses single quotes (‘) for initial quotations, then double quotes (“) for quotations within the initial quotation.
EDIT: Though if so, he's not doing so later in the article.