I can accept "in the thick of the crowd," not, however, in the sense of “in the middle of the crowd,” but insofar as the crowd is conceived as a seething, turbulent mass of people—the use of “in the thick of” precisely conveys an active element in the situation described, not necessarily intentional action of course in this particular case.
P.S. On second thoughts, far more likely is "thick" is being used here in a different sense again (OED's sense a. "The most densely occupied or crowded part (of a wood, an assemblage, etc.).)" Again, the sense is not the unqualified "at the center/middle of." That overlaps with your #12. Notice that OED distinguishes between senses. And I'd argue that the instrumental terms in OED sense a and your #12 definition are "most densely," "crowded," "busiest," "most crowded." I don't think the second part of your lexico definition actually accords with usage; or at least it's misleading: while "thick" and "middle" are arguably interchangeable in a phrase like "in the thick of the crowd," that doesn't hold for "in the middle of the night."
M-W Unabr. likewise distinguishes two relevant senses of the noun "thick":
1 : the most crowded or most fully occupied part : the densest concentration "we came around a turn into the thick of a mob of yelling people — Mollie Panter-Downes" "wide-reaching branches and a thick of leaves — Padraic Colum"
2 : the most intense or most active part or stage "the thick of battle" "major producers … are in the thick of this trend — Wall Street Journal"