Dr Michael Lin, the author of that PDF on COVID that was posted here a while ago had this to say:

"A T-shirt is 79% as effective as a surgical mask. So if someone is coughing next to you and you can't escape, breathe through your shirt sleeve. Any filtration is better than none."

[1] https://twitter.com/michaelzlin/status/1241052444798279680

I wouldn't rush to assume that it was part of a deliberate deception. People assume the function of an item from its intended use all the time and surgical masks genuinely don't provide that much protection from some viruses like measles or chicken pox. I think it's just a case that "they don't protect you" being the unexamined received wisdom and people just not bothering to look at actual studies.

I wish this was true, but I saw this claim made by more than one public-facing scientist that I’m certain knows better. If we had started adopting mass mask wearing, we could have helped reduce the spread and had people geared up to make masks for frontline healthcare workers.