Marketo doesn’t designate another san-serif font if the first isn’t found?
Not if the final rendered font stack is what you posted, no.
I don’t use the Rich Text dropdown to select fonts as it clearly selects fonts that aren’t present on all clients (same, by the way, as the Rich Text editor here on the Community, which isn’t a Marketo product!).
Do I have to add additional code each time I want to use that font (it will be often because they picked it for branding.)
I’d recommend you don’t have them choose a font from the WYSIWYG editor at all. Manage everything via CSS. It makes for consistent, predictable branding. The fact they chose Arial Black without testing cross-platform is a sign they need more lockdown, IMO. (Even if there were a sans fallback, that’s still breaking the brand on one big OS.)