Term for this category of counterintuitive "facts"

It sounds like an urban myth to me.

Urban Myth: a humorous or horrific story or piece of information circulated as though true. (Google Online)

"No, that's just an **urban myth****...." as per the OP

Mythinformation (or sometimes myth-information) would fairly neatly fit your description:

Myth-information

NOUN Information which is widely held to be true but which is in fact flawed or unsubstantiated; common knowledge based on hearsay rather than fact.

Origin: 1960s; earliest use found in Science. From myth + information, probably punningly after misinformation. (Oxford Dictionaries)

So, from your example, you could say:

No, that's just [a piece of] mythinformation.

It's not a very common term, but I think it would be pretty easily understood.

A QI Fact (recognisable in UK if not abroad)

In reference to sort of facts on the television show QI. QI comes from Quite Interesting and QI's creator John Lloyd said that he started collected interesting and unlikely facts just for small talk and for curiosity and then created a game show out of it.

This, unlike the others, does not have the connotation of its truthfulness bring questionable, just unlikely.

Some examples would be:

Ladybird orgasms last 30 minutes.

Whales can't taste anything but salt.

A newborn baby sucks in air with 50 times the power of an adult.