Commentary Purg XV 115-117

The literal sense is not difficult: Dante was not seeing that which was present before his fleshly eyes; from that point of view (the merely physical one) he is delusional, is seeing what does not exist, seeing erroneous phantasms instead of what his physical eyes would report.  But such 'errors' as these are the very heart of truth, or, in an extreme case of litotes (deliberate understatement), are, at the very least, 'not false.'  See the discussion in Barolini (Baro.1992.1), pp. 151-53.