Why would an SDK be secret?
Having access to the SDK doesn't give you unfettered access to publish on their stores. It only allows you to write and compile code targeting their hardware/runtime.
TLDR A game market flooded with shovelware crap is a disaster for everyone.
Modern day consoles intentionally add hurdles/friction/cost to publishing in order to weed out the less serious studios.
And in the meantime, there's already tons of crap shovelware on the Switch storefront. Nintendo doesn't filter for quality.