They've been conflating US policy with Trump's personal motives since this started, even when they were stating it never even happened.
Guiliani has been quite specific that he was representing Trump personally and not the US government. It is quite clear that personal motives for Trump are the genesis of the whole deal, and his methods were at the expense of the usual diplomatic apparatus. So any possible alignment with actual foreign policy is no real defense for Trump. Of course, given that the allegations are Russian propaganda that Trump et al were repeatedly warned against taking seriously, there isn't any legit foreign policy stance either. That's easy to demonstrate as the only foreign policy Trump has shown any interest in whatsoever with regard to corporate in Ukraine is investigating Biden and alleged 2016 election interference (also disproven and originating from Russian propaganda). Those 2 things, if believed, would represent only a small subset of corruption in Ukraine, so someone actually motivated to address Ukrainian corruption would show evidence of involvement in a whole lot of other activities. And any rational person whose motivation is pure is going to keep themselves furthest away from obvious conflicting interests, not jumping headlong into them exclusively.
Anyway, the bigger issue is that Trump's defense highlights supposed legit foreign policy interests when it benefits them and alternatively Rudy's role as not being part of the government when it benefits them. Those 2 things are incompatible with each other.