I think it is only a bombshell in the sense that a reputable news outlet is now finally publicly analysing his presumably real tax returns (they’ve not even fully completed that analysis) and, depending on how the information conflicts with other private understandings, the implications of it could be considerable for Trump and his businesses.
As you, I, and many others have said over the last few years, there is a reason he did not want these released, and even though anyone paying attention up until now will not be surprised by their content, their merely finally being made public (well, relatively public) is a very big event for many reasons.