I feel like it's more of a vibe (that's mostly died out by now) rather than anything quantifiable. Red's the only Champion-esque trainer whose entire characterisation is 'battling Pokemon and being the strongest', since that's the only context you see him in and he has no dialogue, so imagining that he's canonically the strongest is almost a pity thing. After all, it'd be kinda sad to spend such a long time training in complete solitude on a mountain too dangerous for anyone without 16 Gym Badges to approach, let alone climb, only to lose to someone who has other hobbies, responsibilities, and/or personal relationships.