I think that what you mean and that what you are right about, is that if you attend two opposing partisan events, then you are being fair. No argument. But at each point in time, when you speak at ANY event, you are saying that you think that event is legitimate and are supporting its conduct. You are effectively being partisan in that moment.
But a person who got up to say even a prayer at both the March for Life and the March for Women, while being “fair”, would also be somewhat mentally and morally defective. Unless the speech or prayer condemned definite obvious evils or whatever is really wrong or called for repentance of those things, and in some way attempted to speak vital truth in a spirit of love, whatever is said is taken as endorsement, whether you want it to or not. “Partisan” means that the event is meant to support and shore up the organization, in this case the Democratic Party. In that light, it is irrelevant if non-Democrats are allowed to speak. So you are technically right that non-Democrats are allowed to speak, but it is irrelevant. They are not allowed to condemn the Democratic Party. Ditto for the Republican convention.
At the moment, there are things wrong with both major political parties. I would condemn Republican evils as well, but at the moment, it is a case where, while the Republicans would put us on a path that would tear apart our nation in fifty years, the Democrats will do it in ten. They are insanely foolish to promote racial division in the name of fighting racism AND think they can hope to hold the nation together.
On that basis, I say that it is bad and unwise for our leaders to stand up at ANY partisan event, but I think it worse to stand up at a Democrat one. Either way, for the sake of avoiding division and scandal in the Church, our hierarchs should avoid all appearance of supporting evil positions and ideas, even in the desire to oppose other evil ideas.