In Hong Kong dried seafood shops there is an item that goes by two names: 魚唇 which direct translation means “fish lips” and 翅裙 (the “skirt” or tail of the fin). The texture is close enough to shark fin but is of much lower grade and include the skin of the shark’s tail, and of course if one braises it properly it has collagen and cartilage. Once they are dried, they firm up and become more solidified, and thus appears as whole rather than individual threads (like a pomelo).
It is possible that is the substitute that is being used, or it could be not from a shark like what was already said. Easiest thing to do is to ask the restaurant to clarify (bring a fluent Cantonese speaker with you who can ask directly if it is 魚唇 / 翅裙 and from what fish or shark varietal.
Personally I would rather have braised fish maw (the floaters) which have a ton of collagen, using the same superior stock braise (mature chicken/Chinese ham/dried scallops, cornstarch thickened) with goose or duck web, mushroom and abalone with mustard greens instead, way more umami and far more delicious. Or the “vegetarian” version of braised pomelo skin with dried shrimp roe (with the same style braise stock)…but I don’t think anyone in the US can do a proper version of that…