That’s not how partnerships work. A partnership is a single legal entity, and employees are employed by the partnership, not by individual partners. While these huge law firms certainly have some subsidiaries that are separate legal entities, especially for the offices in foreign countries, they never broke themselves up into a bunch of legally separate units that each had less than 49 employees. Small law firms may have done some of that, but for a firm listing dozens of partners on a single letterhead with the partnership name on top, and a single address, and the support staff for those listed partners numbering in the hundreds, no way was it feasible. Much easier and cheaper to hire some deadweights, both as associates and clerical staff, to fill up the EEOC quotas.
Lawyers write laws to aid their professional fellows evade the requirements that affect all the rest of us.