No, mystery and speculation is not genius marketing when it comes to food, or FMCG in general. The consumer markets for FMGCs are pretty conservative - we tend to gravitate to the brands and flavours that we know and like and are at the right price point. That's why it's surprisingly difficult to launch new 'versions' of existing brands - a white chocolate version of a beloved chocolate bar for example. There are years of taste testing, blind testing, focus grouping and marketing before the product launches. And lots of them fail or founder (Vanilla Coke, transparent Pepsi. Vanilla Coke does well in CEE, not so much Western Europe). In general we are a lot more conservative and risk averse with food & drink then we like to think we are.
Genius marketing in this sector would be teasing exciting, delicious looking goods that are well branded and have an element of uniqueness, whilst also reassuring potential buyers that the taste and mouth-feel and price point won't be hugely different to their favourite current brand.
Mystery and speculation has a place in marketing where you already have a very clear brand in place, and are seeming to depart from it. A really good example of this would be Beyonce's team's roll out of Cowboy Carter. Basically it's a strategy which can work for entertainment, media and experiences but not FMCG.
My only other thought is that the Dolly Parton branding works because her personal brand is well established and also very layered: hence the brand partnership has an ironic wink (working class / trailer park / country aesthetics) as well as being fun and heart-warming. You need the legendary status or strong branding of a tried and tested superstar to do that.