I reckon the band have been called "goth" by people before, my brother is always being called it and he wears horrible brightly coloured shirts (he looks more like Weird Al than anything...) because it seems to be a stereotype for any "non bling swingin' chav" around here. So I'm sure Opeth have been called it before by ignorant people.
And I think its just their dark BMesque style that you are talking about. Most DM artists sing about death (duh) whereas Opeth used to be a lot more about journies, inner turmoil, natural imagery, all of which is "very gothic" by todays meaning of the word (and is also what a lot of BM bands sing about when not sacrificing things to satan and blaspheming against god :Smug: ...) so Opeth is thoughtful and emotional rather than gory and macho (like Cannibal Corpse for instance).
This could appeal to todays gothic culture, as they like to be seen as in touch with their emotions, like nature, and are usually into inner turmoil (nowadays usually whether they will make it to the mall before it closes or something epic like that...
) and also supernatural stuff like ghosts (MA,YH and obviously GR now...) so there is a lot of stuff in the Opeth catalogue that I know "goths" would enjoy.
Did Opeth intend to appeal to "goths"?
No, not at all.
Do they appeal to "goths"?
Yep.