Sure. But we don't have to tell them that. You could truthfully tell the dwarves that the Emperor would prefer if the tunnel was done in secrecy until it was ready due to internal and external diplomatic considerations as well as possible Vampire/bandits interference in the project. Hell, you could even point at the fracas between the Slayer Keep and their channel and one of the Electors(was it Ostermark?) as the kind of thing the Emperor would like to avoid as a deflection.
Unless the threat Marienburgh is making is a public one (unlikely), the dwarves would just grumble about umgi political stupidity and ask for some minor concessions for making their new project in secret due to stupid umgi sensibilities. As long as we don't mention Marienburg, some of them might catch on, but those are excally the kind of things that dwarves don't talk about.
Couch it as the Empire being ashamed of not being ready to fight that fight right now, and the dwarves will pretend no shame is happening at the speed of Formula One drivers.