The Hydra was known by a less familiar nickname: the dragon that's weak against other dragons. Its deadly venom, feared by all, had no effect whatsoever on its fellow dragon species. One scholar even speculated:
Perhaps the reason it's immune to its own venom lies in the very nature of dragons.
In other words, the Hydra's evolution was flawed; it developed a poison that was harmless to all dragons, not just itself.
Of course, such theories didn't matter much to Sylas. What mattered was that, if he had the dragon's traits, he wouldn't be affected by the venom.
To any dragon, that infamous poison is little more than harmless fluid. If he had faced this before his awakening, Sylas might not have been so confident, as the dragon's bloodline had been dormant for so long that the Corleone clan was barely distinguishable from ordinary humans. But now, he was certain—the Hydra's poison wouldn't harm him.