His comment was taken as indicating that it is a red line issue for the SNP and could prevent them entering coalition. What he actually said was that SNP MPs would not vote for it - not the same thing. The key is that as part of the answer to the previous question, Salmond had said that he thought a coalition unlikely. I agree.
The lesson of many coalitions around the world is that the minority partner suffers the backlash. That has certainly been the experience of the LibDems. For that reason, I doubt that any potential post-election deal would include the SNP in a formal coalition with ministerial office and a shared programme - that would bring Trident into play as a red line issue.
There is also the issue about a ScotNat minister controlling an English Department, given their understandable interest in Scotland.
My bet is that any deal with the SNP would be formally on the basis of 'confidence and supply' only. Thus the SNP would promise to support finance bills or other confidence matters, but would not have a shared programme and other votes would be taken as they came.
This would allow the SNP to vote against Trident. Any minority Labour government could rely on votes from the Tories or even the LibDems to see any Trident vote over the line, should they decide to continue with the project.
Budget issues - even those specifically for England, actually have an impact on funding for the devolved nations as they affect the calculation of the Barnett formula.
In short, Trident is not a dealbreaker for any post election Labour deal with the SNP.