Is freedom a threat to democracy? Yes, it can be. Freedom to go around killing your political rivals and their supporters is definitely a threat to democracy. Absolute freedom is anarchy, and it's horrible for anyone who is not the most powerful. It's great for them, though.
That's a hyperbolic example. But the freedom of corporations to completely capture political parties and wrest the nation's tiller away from the people and hog it all to themselves is definitely an existential threat for democracy and our well-being. To allow their absolute freedom is a threat to our basic freedoms.
Therefore if you want a properly functioning democracy you will need campaign finance laws which are inherently restrictions on speech. And even campaign finance reform might not be enough now that we are in an era of constant propaganda that is able to lay the ground work of influencing people outside of election cycles.