oh no

oh no I apparently feel very strongly that you're wrong here

You're right that m-dashes should be spaced, of course. But there's a big difference between an m-dash and an n-dash, and you used the wrong one in your example. An m-dash, like a semi colon or colon, is for separating two related clauses — there's never at time when you should use two in the same sentence. Whereas n-dashes are used for parantheticals –sub-clauses that can't stand on their own– and should, like round brackets or quotation marks, have spaces on the outside but not the inside.