Assuming this isn't a joke, he means if you were to build something with their api, it is free to you up to your 100th user.

I believe that would 100 users for the service that you would write using their api. See bottom of this page: https://plaid.com/products/ (playing around with the api to help make a self-use budgeting app - under the current terms, this use case would be free).

I think we may have set a record if that was the case...

But the other commenters are correct - the first 100 additions to the api are free.