Ta'anis 3b - No Wind, No Dew

The gemara says that since wind and dew are never withheld, even if one declares that Hashem withholds wind and rain, it does not mess up their shemoneh esrei. The Poras Yosef (in the back of the gemara) says that so long as something is clearly an obvious falsehood, it does not mess up shemoneh esrei. Therefore, if one says ya'aleh v'yavo for rosh chodesh on a regular day, they do not have to repeat shemoneh esrei.

I am not convinced by his comparison. Although G-d does not withhold dew and rain, he at least has the power to do so, and in the context of the praise that he is the "almighty" it is not a 'sheker muchlat'. But, to declare today is Rosh Chodesh, when in fact it is not, may be worse. Any thoughts??