"Ah, by van Baelsar," he muses. "No wonder, then."

If there is any incongruity to be found, it would be in that very comment. The man standing before Nero right now, after all, is surely too young to know of Gaius as a legatus - never mind that Gaius probably hadn't even been born yet.

It's a detail Emet-Selch doesn't seem to care about. He's already starting to climb back up the slope to rejoin the masses.