Are they being prevented from sharing or expressing what matters to them in any way? Because Facebook isn't amplifying their preferred sources via the trending section?
Yes, according to the allegations implied in the letter. If Facebook penalizes topics which don't agree with Facebook's bias - that certainly would be preventing some of their users from expressing what matters to them. I don't follow your second question about amplifying.
Basically, people are still free to post whatever they want and their friends and families will still see it -- they were just restricting their "Trending" section to non-conservative news sources. So their freedom of expression was upheld but FB just wouldn't amplify certain sources.