Opposite here. Apart from Git’s command line output, tabs are and always have been 4 spaces to me. And while your textarea example might work, I don’t even know how I can enter a tab into a textarea (without copy-pasting).
I would actually say, if the code span only contains whitespace, leave everything there. How else am I supposed to tell you otherwise in markdown, that you need to add two spaces after a line to insert a hard line break (for example). I’d say that when using spaces explicitely inside code-quotes, then the author likely has a reason to insert all those whitespace characters.