There's way too much gatekeeping going on here. Your light bike workout on a bad day is as much of a workout as a high intensity treadmill run on a good day. Both are very much proper usages of gym equipment. Doing something suboptimal is always better than doing nothing.

people suck

you can absolutely get a great workout in on the bike while not having to go through the effort and tax of moving equipment, racking/unracking etc

just have a seat and go as hard or as light as you want, with the ability to instantly change intensity in response to how you feel. no need to think - just sit there with your head down and pedal hard

It's almost like people intentionally misunderstand things. that, or they simply don't have the experience and so weigh in on things that they don't know about

however that person's comment that indoor biking isn't much of a workout is just blatantly false. It's as much of a workout as you want it to be. People just don't know what it feels like to be holding hundreds of watts for a long time, and that that's what's required for a solid workout

I rarely use the bike these days but generally if you're using a stationary bike for exercise, you shouldn't be sitting the whole time.