I think this is true, but not necessarily the reverse. That is, free will requires sentience, but sentience doesn't necessarily require free will.

And I think that is actually how it is in reality.

Sentience is simply the ability to observe or witness feelings and sensations. The fact that I witness feelings and sensations is in fact the only thing I know for sure, which is to say, one's own sentience is the only thing anybody knows for sure.

But the more you think about free will, the more you realize that it's just a problematic idea. It's kind of like how any movies involving time travel always have some plot holes. Free will is kind of like that.

Here's a video that I think is helpful in understanding why free will doesn't make sense conceptually: https://www.youtube.com/watch?v=zpU_e3jh_FY

I liked this article. You are correct that there isn't any particular thing that will always categorically differentiate humans from an AI.