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The highlighted relative pronoun is wrong; it should be who (Sunday Times, 19 May 2013).

The incorrect whom seems to be the result of the author mistaking it for the object of I think (i.e. *I think whom), when in fact it is merely parenthetical. The relative clause is therefore saying who (I thought) were quite the perfect Hollywood couple.

Why, then, the subject pronoun who rather than the object pronoun whom? Because who is the subject of the relative clause: it stands in for Brad Pitt and Jennifer Aniston, so the relative clause is in effect saying Brad Pitt and Jennifer Aniston were quite the perfect Hollywood couple.