On those older zooms, the macro switch usually was not a separate built-in extension tube. It typically changed how the lens’s internal groups could move so the lens could focus much closer.

In normal operation, the zoom and focus groups move within a limited range. Engaging macro mode unlocked an extra range of movement—often only at certain focal lengths—so one or more element groups could shift farther than usual. That increased the lens-to-film/sensor optical distance and allowed closer focusing, sometimes by moving several groups together.

The switch or lock was there because this extra movement only worked safely in certain zoom positions; outside that range, internal groups could interfere with each other or the optics would no longer function properly.

So the idea is similar in effect to extension (getting the optics effectively farther from the film/sensor to focus closer), but mechanically it was usually done by reconfiguring the lens’s internal movement rather than adding a literal extension tube.