I think that it means the reason for doing where 1 = 1 is sometimes to allow for easy insertion of dynamic queries which can be a security and performance issue. The actual usage of where 1 = 1 doesn't cause the security or performance issue.
Which is exactly what the site says. To insert dynamic conditions. I know that you can use 1=1 for the same reasons as trailing commas. But kinda obvious that this is not the case here.
