I think this happened like 3 times already. Trump announces some sort of positive breakthrough in negotiations - Iran announces they don't have a clue what he is talking about.
I'm not sure if this is some sort of 5D chess, or if one of the sides (likely Trump) is just taking a piss. Or maybe this is some desperate attempt to calm the markets down - if so, it's working.
It's also possible that it's Iran that engages in negotiations, but tries to appear tough and uncompromising in front of their nation, to not allow for defetist mood, but I think this is much less likely than Trump making the stuff up.
I think this is just a tactic in modern warfare, where media is also a strategic weapon
This is most likely a propaganda and face saving strategy for both side. Trump is most likely trying to save face because they are a superpower. Besides that as you mentioned, it was also an attempt to calm the market.
Meanwhile, Iran also does not want to be seen as weak and does not want to beg. Let's not forget that their conservative faction, IRGC, is very aggressive and willing to fight to the bitter end. Therefore, it is not surprising that they issued such a strong statement.