John Allen makes sense:

Why the explanatory vacuum? The answer, at least implicitly, seems to be the following: This pope is his own gloss.

In other words, precisely because this was Joseph Ratzinger, it is difficult to imagine that the prayer at the Blue Mosque, at least on his side, had anything to do with a relativistic approach to religious belief. It was unnecessary to slap a warning label on the event saying, “Syncretism is hazardous to your faith,” because the mere presence of Ratzinger communicated in a flash all the doctrinal caveats that form part of his understanding of such events, including his criticism of the 1986 Assisi summit.

Had this been another senior Catholic official — Cardinal Roger Etchegaray, for example, or Archbishop Michael Fitzgerald, both known for more irenic views on Islam and other religions generally — fierce debate might have been unleashed about the theological meaning of the prayer, and one can imagine the Vatican issuing a “clarification” spelling out all the qualifications.

Yet this wasn’t Etchegaray or Fitzgerald. It was, instead, the very man who identified a “dictatorship of relativism” as the central challenge facing the church one day before his election to the papacy.

None of this is to suggest that the prayer didn’t come as a surprise, given Ratzinger’s background. (In fact, prior to the pope’s own comment exiting the mosque, the press corps traveling with him had been locked in fierce debate over whether to characterize what had happened as “prayer” or “meditation.”) But it was precisely that background which, one imagines, emboldened Benedict to accept the invitation to pray, assuming that it would be seen in the context of the totality of his life and thought.