It was full health with one unit elite, admittadly they had a massive stack of macemen in reserve, but only one attacked...
Was it:
a. One maceman that took all the points?
b. Or that huge stack finally killed the tanks after like 20+ attacks?

If a. Then you are extremely (extremely, extremely!!!) unlucky.
If b. Then you're unused to the new stack system in civ3 with huge armies, you'll need huge armies for yourself now.

But still, even an understrength tank division should just roll over a bunch of guys with maces.
Well, a few tanks ran out of fuel or got some parts broken. The macemen attacked the stucked tank, killled the crew and took over the cannon tower and shot a few other tanks apart. Your own incompetent crew helped by accidently shooting some of thier own tanks when they panicked.
It could happen.

And besides, when we're on the realistic side, how often had the tanks vs macemen happened in history? Never. Could it happen? Yes, but only if you encountered a new continent that hadn't had any contacts with you before and then they would have stolen most of the technology in two turns (=atleast two years). The outteach in all the civs series is extreme and that the comp use outdated troops (instead of upgrading) doesn't make it better.