Earlier quoted context omitted.
why tho? it's just an alternate alphabet/set of symbols.
Because its generally expected that models only work 'in distribution', i.e. they work on stuff they have previously seen. They almost certainly have never seen regular conversations in Base64 in their training set, so its weird that it 'just works'. Does that make sense?
People use Base64 to store payloads of many arbitrary things, including web pages or screenshots, both deliberately and erroneously, and so they have almost certainly seen regular conversations in Base64 in their 10tb+ text training sets scraped from billions of web pages and files and mangled emails etc.