Coffee, beer or a cocktail can certainly be tasty, so I don't think it's about tea being liquid. I think it's more that tea... just doesn't have an intense taste. (If it did - like a fruit tea - you'd say tasty, wouldn't you?)
As far as I can tell 'smaczna herbata' is OK Polish, but the translation 'tasty tea' is odd. Still, I think it should be accepted to prevent people having to guess how it should be said.
I generally agree with people who complain that one would not say this; on the other hand, I don't think that the alternative expressions really mean the same thing. I might say ‘tasty tea’ if for some reason I wanted to emphasize that the tea was tasty and not just nice or good in some other way or for some other reason. PS: from the U.S. in case it matters.
Something that is more of a 'social meeting with tea' would probably be called using a diminutive: 'herbatka', although 'herbata' itself is possible as well.
But it's not that it's a substitute for dinner, I'd say, it's rather a fancy meeting with friends, like when in Toy Story Buzz Astral is dressed up as the countess and having tea with dolls.