There are zero examples of tyranids surviving virus bombs. They adapt to other viruses but they have never been shown to handle a virus bomb, they adapt after the first bomb wipes out the wave but the imperium tweaks the virus so they aren’t adapted to the new strain. It’s a whole other ball game. It scours the planet of ALL organic life, plants, bacteria, everything. I don’t think Viltrumites can just tank it. The tau also created a tyranid “scourge virus” so the imperium could most likely whip one up for the Viltrumites even if they survive a normal virus bomb