It’s actually an open legal question. Actual legal scholars have argued both ways on it. Yes, there is was a deadline in the act Congress passed to send the amendment out for ratification. But the key is that they didn’t include that deadline language in the text of the amendment itself. Some other amendments have language in the text of the amendment that places a deadline on ratification. That is the crucial difference here.

A good argument can be made that Congress can only propose an amendment or not. They can’t attach a bunch of extra provisos to the amendment process. Congress can’t confirm a justice to the court and apply a bunch of conditions to that confirmation. If they want to have a time limit on the ratification of the amendment, the time limit should be in the actual text of the amendment itself.