Yes, they banned only the mining activity. But it will be too early to claim that it will not be having any impact on the market. Because Iran had a considerable share of the mining power. And I am more worried about the miners there, who made huge investments in terms of mining equipment and cooling systems. If they are forced to suspend the operations for 3 months, then they will be short of cash. These miners will be forced to sell whatever reserves they have, and it will further push down the Bitcoin prices.
Your opinion is correct, miners will dump their crypto holdings to maintain warehousing, staffing and other costs. The market can freeze for a short time. I trust they will pass soon as they have been mining Bitcoin for a long time so they will be fine.