I agree with you that "authentic" is a loaded word, and I usually don't know what people mean when they use it, especially when they ask to be taken to "authentic" bars on my tapas tours. Will they be disappointed if I take them someplace without bullfight posters and sawdust but that serves some of the best jamón in town? I used to work at a fabulous tapas bar in Bristol that used fresh "Spanish" ingredients but ended up serving tapas with a twist - and all totally delish. Meanwhile, as you say, sometimes totally "authentic" tapas bars in Spain can be crap.
Me? I keep hoping a decent Thai restaurant will open in Seville...