If Doppio tells the Legendary Seekers about his experiences, he'll absolutely become their new best friend. In fact, he'll probably have to deal with everybody in the seekers wanting to talk to him and recount his experiences. Some would definitely be telling him their theories on Arceus, some of which (all of which, lets be honest) are pretty out there.

Considering Doppio has experiences with multiple legendary pokemon at this point, he'll get elevated within the organisation to the position of Legendary Expert, which comes with exactly zero benefits, except getting to look at even more blurry pictures of what people claim are definitely legendary pokemon.