Just like how you told me earlier to look on hltv and reddit, you show me where the legacy is what defines an S tier event. Because everything I could find, was all opinion as there's no strict definition of what an S tier is, only what general communities consider S tier.
You're right, the definition has never been challenged. It's just been considered that S tier events are the biggest events during the year. This tends to be linked to the level of competition, the scale of the event, prize pool, production, etc etc. it's been like that since star craft in the early 2000s. The legacy means nothing, it comes along with it.
I agree that so far valorant has only had S tier events that have been apart of the vct tour. However that doesn't mean that S tier is only restricted to first party events. I think apex is the only esport where they only have official events as S tier.
The definition of what constitutes an S tier event is also community driven, and so far you're the only person I've seen only define it as official first party hosted events, that stick to a specific criteria.
I wont lie that first party events tend to have a larger impact on a players legacy, that's undeniable. However that doesn't mean those events are the only S tier events.