No, it is not a compromise of your beliefs to sit down with other Christians and ask what do we agree on and then state those beliefs. It is compromise when you try to pretend that you agree when you don’t or to say that the differences do not matter. That is not what these people are doing, they make that clear.
Do you think its compromise for Catholics to sit down with Eastern Orthodox and Protestants and say what can we agree on and lets write this down on paper, even if in some areas its very general agreement? I don’t see how you can compromise your beliefs when you’re not giving anything away, you’re just stating what you hold in common with other Christians.
You’re not waving anything off as unimportant by writing a general statement of belief. You’re just not going to wade into highly specific doctrines over which different traditions do not agree.
Protestants are divided over many things, but they are also united over many things. There is broad consensus among evangelical Protestants over the basics of Christian faith, as is seen by this confession. There is no need for me to “compromise” my beliefs to acknowledge shared beliefs with Protestants, Catholics or any other Christians.