When Twitter's defense against the government's request is because of the ideal of free speech, not the First Amendment but the ideal that it is based on that predates the First Amendment, then it is worth pointing out the double standard in Twitter's own behavior not upholding that ideal.
There's no double standard though. This is like saying: "how can a newspaper say they're in favor of free speech if they're not willing to publish any story that's submitted to them?"
Twitter can support free speech by advocating for their right to control their platform, and for other people to have the right to create their own platforms and put content on it. The government using strong-arm tactics to unmask critics is a threat to free speech through any medium.