The phrase "to believe this myth" means that someone thinks something is true even though it's not supported by facts or evidence. It indicates that they have a mistaken belief in something that is not accurate or real. Full definition
Don't believe the myth of «keeps baby drier» that you see advertised. (greenmountaindiapers.com)
Don't believe the myth of the single credit score. (lifehacker.com)
The pet food industry has been touting the false dangers of bones for decades and many people still believe myths about the danger of feeding raw bones. (peterdobias.com)