I get the same result, using the Windows Version and patch 2.08: He does not have the 'assassin' ability icon in his description, but does get the option to assassinate.
I would agree that using the pretender as an assassin is not really worthwhile most of the time, so would not be unbalancing.
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Thtrap it to the bench and put a good thick bolt of lightning through it, that'th our motto. That'th how you tetht thomething!
- Igor