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When I lived in Bern, 'Schriftdütsch' and 'Hochdütsch' were synonyms. My dad once went to a meeting where the conversation was all in Mundart, until the other attendees realised he was there and apologised. 'Kein Problem,' he said, 'ich verstehe Bärndütsch.' The meeting continued, but when they'd finished and were getting ready to leave, one of the people came up to him and complimented him on his German. 'Aber nur noch ein Ding,' he said, 'das war kein Bärndütsch, sondern Baslerdietsch!'
This explains:
https://www.justlanded.com/deutsch/Schweiz/Landesfuehrer/Sprache/Sprachen
Schwyzerdütsch has no official status but is de facto what is meant by "Deutsch" in Switzerland, with the exception of the use of Hochdeutsch as the "Schriftsprache". Perhaps someone told you about this term and it caused confusion; it doesn't mean that the other dialects aren't written down, just that Hochdeutsch is used for official use--official documents can also be known as "Schriftstücke" or "Schriftsätze", which I think is where the "Schrift-" bit comes from.