It actually made me able to watch shows in Spanish. It just happened as a result of me doing Duolingo basically like a game.

I was still in the middle of Spanish course when I realize I can sorta kinda watch and understand some shows, so I watched. (The watching itself then made me progress mucj further, but it would not happen without duolingo).

I use Duolingo to learn French, for a few years already. It definitely can bring you up to A1/A2-levels of proficiency (at least for French), which is definitely a solid starting point to engage with the language further. In my case, I've started to take weekly evening-courses. If I started another language, I probably would start again with Duolingo for the super basic stuff, then start to learn vocabulary with Anki, and then start with some paid, organized course that guides me through the more complex parts.

I still use Duolingo almost daily to have some continuous language exposure, for which I still find it useful (especially as the gamification helps with staying engaged). It has its limitations but it does help me. Just to give a bit of a counterpoint; I find your statement a bit overly broad.

If Duolingo hired someone to be a plant and post under his real name on HN for over a decade just so one day he could make a lukewarm endorsement of the product in a reply to another comment, I'd have to question their business sense.