If it's bad and we don't avoid it = potential disaster
If it's no problem and we avoid it = Some unnecessary losses
If it's no problem and we avoid it = No problem
Currently, considering past patterns, it looks like PFAS are problematic and the potential cost of failing to mitigate could be very high. So being more cautious is the rational solution, even in the face of uncertainty.
Going by the pattern of the comment, I would assume this was a typo. Maybe they intended to say:
"If it's no problem and we DON'T avoid it = No problem"
Lots of substances may not need to be tested if they occur frequently in nature as we can assume that life has already mitigated the problems that they may or not cause. If we start to widely distribute these naturally occurring substances, then we need to re-examine what effects they may have (e.g. lead occurs naturally, but putting it into the air produced a very harmful effect on human development).
If it's a new substance that doesn't already occur in meaningful quantities, then we need to be very careful before we start putting it into water supplies as that has the potential to disrupt a wide variety of life and habitats. To merely consider it not harmful due to lack of testing is really foolish.
It's an assumption that this will be similar, but sure.