Well... a stormlord is an Imperial Guard superheavy, so it's an available transport to Imperial Guard. You couldn't take it if you were playing Space Marines straight. You could, I suppose, but it would be an Imperial Guard vehicle for the mixing armies rule.
The Plague Tower is a Chaos super-heavy (specifically Nurgle), and Epidemius is a Chaos character (also, Nurgle). So it would make sense that you could transport him - just as you could transport Commissar Yarrick in the Stormlord.
I'm sure not everybody played straight by the rules on either side, on this scale it's expected that rules would be overlooked - purposefully or not. But the mixing armies rules was shoved down our throats multiple times - it's something good to remember.