The thing is that intelligence is kinda orthogonal to the ability to feel pain. Bacteria don't seem to have a nervous system, so that would be a place to draw a first line.

I feel that this is a click bait to attract employees. The writer links to his startup on his profile (which Medium shows right next to the article), where you can see that he's looking for founding engineers. Rust has been a trending topic lately, which attracts relatively experienced engineers. He mentions that he made these experiences more than 2 years ago, so it's notable that he waited so long to write about it. The slightly noisy "ex-google" and "ex-apple" mentions sound like advertisement.

And, well, the usual, "learning curve" and some largely hypothetical concerns about hiring. I developed a complex app with Rust with very high time pressure and with developers that didn't have any previous Rust experience (I didn't have much of it myself), it was risky but it worked out well, so I know first hand that it's possible. And we are not "ex-google"!