Quite simply today’s halacha plainly states that it is forbidden to speak lashon hara even if no harm was done, and even if you somehow knew no harm would be done.
Unlike other sins between man and his fellow which are only a sin if it actually happened liked stealing, overcharging or interest, lashon hara is a sin even if no harm what so ever is done. The proof is the fact that the gemara in Erechin 16a says the coat the Kohen Gadol wears in the Beis HaMikdash with the bells and pomegranates is an atonement for the sin of lashon harm if no harm was done. If harm was done then the person received tzaraas, (spiritual leprosy.) There would be no need for an atonement if harmless lashon hara wasn’t a sin. It must be that the very fact a person just speaks negatively about his fellow Jew to his own benefit is enough to be a sin even if no harm is done, which Rabbeinu Yonah in his Shaarei Teshuva also points out.
Furthermore we saw why Miriam was punished with tzaraas even though Moshe didn’t take what she said to heart and it didn’t cause any skirmish or hard feelings whatsoever when Miriram told Aharon lashon hara that Moshe separated from his wife and the Torah said Aharon responded to her words. Rechilus is deserving of tzaraas if it effectuated some kind of skirmish to start, ill-feelings between two people. But lashon hara is deserving of tzaraas even if the speaker caused the listener to start talking about the situation, even if he doesm’t say something negative or tries to defend the person spoken against but point is it made an impression. So because Aharon started talking as the Torah say, after Miriam told him about Moshe, she was therefore derving of tzaraas. Nothing else was said after that and that is why Aharon never got tzaraas. Whatever he said had zero impact on anything.