Correct me if I'm wrong, but I don't think that there has been an instance of a Gym Leader openly admitting to holding back or purposefully using weaker Pokemon. I think the case is that the Gym Leaders are all tough across the map and not solely based on what order we fight them in the games. In the anime, it is implied or outright stated that you can start your Pokemon journey from any city. So I would guess that you would be able to challenge that Gym Leader as your first one even if in the games you "supposed" to take them on at a later time. Sure the Gym Leader can turn you down for lack of experience and ask that you come back with a few more badges, but I think that's more to do with the fact that badges are sort of an indicator on how skilled you are and if you come back to that Gym Leader that said you were inexperienced with 4-6 badges, they would be able to take you more seriously as to them, they are pretty hard to obtain and getting just one of them is a rough go.
They even say that of the hundreds of trainers that make it to League, thousands more don't even qualify due to how hard it is to get 8 badges. It easy to see that Gym leaders don't hold back, no matter who they are battling. Otherwise there would a be larger pool of trainers that make it to League due to Gym leaders holding back on them.
My guess is that the attitude the writers are going with. There is no set order in which the Gyms are supposed to be challenged at so there is no need to make one seem weaker or easier than the other. They are all tough and they shouldn't be holding back.
They even say that of the hundreds of trainers that make it to League, thousands more don't even qualify due to how hard it is to get 8 badges. It easy to see that Gym leaders don't hold back, no matter who they are battling. Otherwise there would a be larger pool of trainers that make it to League due to Gym leaders holding back on them.
My guess is that the attitude the writers are going with. There is no set order in which the Gyms are supposed to be challenged at so there is no need to make one seem weaker or easier than the other. They are all tough and they shouldn't be holding back.