Having run a "customer feedback" (i.e. review management) company for 5+ years, we were very cognizant of this rule and its wording, and even in the absence of known penalties, I'd still recommend adhering to its "spirit".
"Bulk" solicitation evokes this scenario: a business with 20 years of customer data wants to buttress its GMB listing by blasting its historical customer database with review requests. If successful, this seldom-reviewed business suddenly gets scores of reviews on GMB in one fell swoop. Even if there's no Google penalty per se, the result smells funny to consumers, especially a year or two down the road when all of the listing's reviews are old.
"Transactional" solicitation is a better model to follow, even if requesting reviews from past customers. Reaching out to a few at a time over time creates a natural, consistent flow of reviewers, adheres to the spirit of Google's policy, and produces a better result WRT to savvy consumers who pickup on this stuff in surprising ways.
