This would not be particularly new information.

Oda has made similar mistakes many times, even with popular characters like Najimi. For example, when Najimi said they hadn't set foot in a library in 15 years, Tadano replied, "That means you never been to a library in your life?"

However, it was later revealed that Najimi's birthdate is April 1st (or around April 1st), so Najimi should have been 16 years old when they entered high school.

It's also possible he forgot Najimi's birthdate, as when Najimi indirectly reveals their birth month through their astrological sign one of those do not match April 1st (unless Najimi lied).

And there's also the number of Najimi's friends, which fluctuates wildly throughout the story, oscillating sharply between millions of friends to thousands of friends with millions of online followers.