That's true - at least, I did when I was a housecat. It was just sort of instinctive. I kept meowing at different pitches until I found one that got my housefolk's attention, and then stuck to that. The cats in ThunderClan meow sometimes, but mostly when our leader is calling everyone to a Clan meeting.

[The rest of the time they're speaking... vague cat language? The books are unclear.]

The Twolegs back home talked, but it was all garbled noises. Here, it sounds to my ears like everyone is speaking cat. Um... in my world, cats also talked a lot about Twolegs being dumb, since they couldn't talk like us, so I guess it's normal to make that mistake.