There is a theory that the character 民 depicts a slaves eye being blinded; this character is recorded back to Shang times where there was ritual sacrifice too. It's recorded in pre-Qin books (see 周礼 in link below) with 人 as 人民 in a way that suggests they are two separate nouns in an unmarked conjunction; this would later have led to the current 2 character word perhaps as the social conditions under which pre-Qin slaves existed disappeared?

https://zi.tools/zi/%E6%B0%91

https://zh.wikipedia.org/wiki/%E4%BA%BA%E6%B0%91

I'm no expert so just trying to reconstruct your professors logic, which seems plausible with my limited knowledge.