I don't know about that case, but if Mahony granted an indulgence he would probably have been in error. Indulgences have to be pre-approved by a church body, probably the pope. No single churchman can suddenly invent a new indulgence and bestow it. I doubt even the pope would do that. The conditions for obtaining the Holy Door indulgence were published well in advance. I don't know if the pope devised and enacted it unilaterally or in concert with some committee of bishops.
It could be possible that Mahony would have the authority to do that, but I doubt it.