Moltres, Rampardos, Ho-oh, Bastiodon and Houndoom learn it too, just to name a few.

Most likely for Thunder Strats or Strengthening a Water Type Ally's Water attacks if you're using Flareon for Support. Although, it might be for Hidden Power Water if a Flareon happened to be graced with those IV/ EV spreads. I don't know if Hidden Power Water or Fire are strengthened by the Type respective Weather Effects.

I just was confused as even though Flareon is a fire type and he is weak to water types... can Flareon learn any other water type moves?

Nope. Not in any Generation Hidden Power was learnable via TM, but, does Water Type Tera Blast count? It IS Water Type when Flareon Terastalizes.

Hidden Power is now Unown's Signature Move, Water Judgement is Splash Plate Arceus only. Flareon can get Water Tera Blast with a Water Tera Type.

This was before gen 5 so the tera type didn't exist back then

I know. So, does Hidden Power Count?

Hmmmm, I am not sure. I will find this out as you got me curious now

Again, the Flareon has to have the correct EVs/ IVs for that Hidden Power Type. Because Eevee is rolled when Bebe gives it to you. So, best to check in Veilstone Game Corner Prize Counter with the Hidden Power Seer. He can tell you want Hidden Power Flareon has. Although, when moved to Scarlet/ Violet, it can be given a Water Tera and Tera Blast gets both a Water STAB, most likely also being boosted by Rain. Plus, Flareon gets Tera STAB with any Tera Type. Whereas Hidden Power is never STAB.