What does "100's" mean? 100%?

It means many hundred, or more than two hundred in the same way that dozens means at least two dozen.

Hundreds? Why not having used 100s?

The better way to express the sentence would have been to use "hundreds".

The writer/ad copyist was <sigh> trying to be clever.

There is a rule for using numbers vs. numeric description (33rd vs. Thirty-third) but I don't remember it exactly so I won't try to quote it here.

I use a word rather than a number unless there is some special reason to do otherwise.

Joe(I'm Number 1!)Nation

Thay shud have done that; your way is right.
Their way is conceptually incorrect.





David

Use a word to begin any sentence and if the number is less than.... something. I think it's 20. So you would write, Three hundred people attended the event. or... There were approximately 300 people at the event. They enjoyed the fifteen food selections.

That rule is unsupported by competent logic.
It is unworthy of respect or compliance.





David