I'm guessing bot authors will start to filter the instructions, minimally starting with s/I'm a bot//g
TBH, if someone posted a job with the instructions "start with I am a bot", I -a human- would reply with that exact sentence. Obviously if I'm interested in the gig. They are a common "CAPTCHA" on Upwork. And it's easy to just follow the exact requirements. As human, as well as a bot. This "experiment" is extremely bad designed. It proves nothing.
I don't think a human who read that would start a cover letter by stating they are a bot.