Note that they do both serve alternative purposes from each other.

Free users can make memes with AI.

The Premium subscription applies to the API only. You can make AI memes for free, but not with the API.

What is the API?

https://en.wikipedia.org/wiki/API
An API is normally utilized to integrate some features into other websites, software, etc.
This means that you could, in some way, have your own Meme Generator.

imgflip.com/api
Here's a list of all Imgflip API requests.