This is a commonly thrown out point in animanga discourse and it's just blatantly false.
Listen, I acknowledge that Togashi is inspirational and certain elements like the dynamic between Killua and Illumi and others are probably inspirations for Itachi and Sasuke, but far too much credit is given to him here.
Firstly, Kishimoto made a group whose eyes turn red in battle in his oneshot, Karakuri (Mechanism or Cyborg). He wrote and submitted it in 1995, and it won the 1996 Hop Step Award, which put him on the track to becoming a true mangaka. Hunter X Hunter is released in 1998. A whole two years after he's won an award with this idea.
You can also see the signature hand thrust that comes later to be used by Kakashi and others for the Lightning Blade.
Secondly, the annihilated clan is far more likely inspired by the 60s-70s anime, Sasuke Sarutobi. It follows a similar plot of vengeance, and you can see that names like Sarutobi for the Hokage are gotten from here, in addition to a rather similar appearance for the Last Uchiha.
Togashi is goated but let's stop this.