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Pikachu had never dynamaxed before so we can't say he was retconned from regular dynamax to gigantamax, and even then it's very possible that Pikachu is able to gigantamax because of Eternatus, which caused Pikachu's electricity to flare up and was the only pokemon on the plane to react.
There is also nothing about Pichu showing up 4 years before the series began to suggest it's a reboot unless you're retarded
