You are correct that a larger audience would be ideal. But in my experience it is not necessary for Signal to work. The main reason is people explore hashtags on Twitter to find relevant content. Yes there is a lot of content posted, but the auto-tweeting function means you are potentially reaching new readers 3 times per day, 7 days per week (with only minimal effort top set up).
Readers can also share your content without following you (this happens with a lot of my content on Twitter through Signal). Additionally, increased activity on Twitter can help build your audience which can be useful for marketing your own content (whether it is on Medium or elsewhere).
You can use the free version for 10 articles (last I checked). I have had a few articles gain hundreds (or even thousands) of extra views months after being published on Medium, from a few random retweets. You only have to set it up once and then forget about it, which I love. There is little downside (as it is free to try) in my opinion.