Spin-off games for Pokémon ALWAYS have a short gestation period from reveal to release. Rumble was 2 months, as is this, as was PokéPark...Conquest was four months, yes, but we didn't get anything really until just before.
Just because reveal and release are close does not mean it's rushed. Games do not begin on reveal, they are developed for ages beforehand and this is no exception
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I know they could have worked on this prior to the official announcement
Of course they worked on it before the announcement. Where else would the footage from the trailer have come from? Games take a LONG time to make. There's no way they could make a quality game in a couple of months.
"Let's just say they're...not okay" - Madara Uchiha