Sentence Correction

When an action is in the past and ongoing, it's supposed to use has/have as the verb, no?

Thanks in advance!

'had long' is used here because it used to be the case until it became clear that the ozone layer is not immune to human intervention. So the appearance of immunity was in the past tense. using 'has long', as in the answer you picked, is present tense and implies that it still seems that the ozone layer is immune, which is clearly doesn't anymore as the hole in the ozone layer is a well known environmental issue.