Having a "most recent login" doesn't prove someone is an active user. I use it about once every two days, am I also an active users? Compare that to WhatsApp which most people use multiple times a day or even multiple times per hour, and you get the picture of how popular or lack thereof Signal is by comparison.
Like I said, a lot of people have Signal, but very few use it as their primary messenger on a regular basis, and more of a "it's just there in case one of those tech nerds who told me to install it decided to message me on"
Yes, that is definitely a non-standard definition of "active user". It's not really a relative term - if you're signed in and sending/receiving messages, you're an active user.
Like I said, a lot of people have Signal, but very few use it as their primary messenger on a regular basis, and more of a "it's just there in case one of those tech nerds who told me to install it decided to message me on"