"I'll show if you fold" can go either way. In fact because it's a common line that seems to have started as meaning they've got a hand can be so easily flipped over to mean the opposite, it's poker, that's what people do, they try to conceal information. If it's easy to manipulate, ignore it.
That's not to say no verbal tells
are worth listening to, if there are inconsistancies in what someone says in various situations, if their tone of voice is different, the direction their head is facing when they say something, it's all worth being aware of.
It's the same with visual tells, as an example deep breathing is such a reliable indicator against someone in their first live game because it's so basic yet after they've played a session or two it's the LEAST reliable tell because it's so easy to manipulate and every half-decent reg, even some terrible regs, have learned to turn it around.