These pillars stand on the Piazzetta, near the lagoon, and were once the official gateway of Venice. They are no. 7 on the map at the back of the book (Afterword p.7).
St Theodore was the patron saint of Venice until he was ousted by St Mark (see bookmark 325). Perhaps this ousting is why Miss Garnet calls him 'spurious.' On the other hand it could be because he committed arson. Or perhaps, since he seems sometimes to have been confused with St George and with other St Theodores, she simply meant that he didn't really exist.
St Theodore is also said to have slain a dragon, to prevent it from devouring the children of a Christian woman who had been thrown to it as a sacrifice. The dragon is transmuted into a crocodile on the pillar, and this association with Egypt is perhaps on account of St Theodore's father having been Egyptian.