Just a few observations, based on my own experience.
The teaching method of dl is based on two things: presenting pairs of sentences, and repetition. When I used to work via the web browser, I had blocked most of the distracting elements of the page, which included the leagues block. Regarding repetition, some years ago users had the option to repeat both lessons from specific units and random general lessons. I used to use these two options quite a lot. In general terms, my learning strategy was to advance one, two or three lessons, depending on the content (vocabulary tended to be easy, but grammar not as much), and then I used to spend several days just doing repetition.
On my cellphone, well, I use the app, thus, I have to live with all the distracting elements Owing to this, I took notice of the leagues and well, I started to compete. However, since my goal is to learn, I keep following my strategy as usual, that is, I do my lesson, and then start to repeat until I feel I have grasped the new vocabulary or characters from the scripts I might be learning (e.g. Arabic). For me, competing is just an extra fun way to do the repeating of my lessons. Since I started I have ended first place at each league, until I arrived at the diamond one, where at that first week someone did 20K points in a few hours; obviously I did not try to kill myself trying to reach her. The goal is to learn.
So, it is not an incompatible either-I-learn-or-I-compete situation. No. Whether competing is detrimental or not to one's learning, depends on one's own approach to it.
By the way, for those eager to compete (and learn at the same time), and wishing to get the most points for the time and effort done (without being subscribed to duo plus), there are many different ways to manage the various resources available, specially when learning three or four languages at a time. Doing 80 XP in a minute or minute and a half is not something particularly difficult to do. I can do 2K in a relatively short period. The most I've made is like 8K in a day.
Regarding learning… to think that by just doing duolingo one will actually learn a language, well, as other participants have expressed elsewhere, that leaves a lot to be desired. Of course, there are some courses that are very complete with a large number of units. Those will certainly provide a good starting point once finished. For example, the English–Italian course is one of them. However, even in these cases, one should always use other resources: web pages, videos, podcasts, books and audiobooks &c. If one does dl along with all the resources available, at the end one will have great platform to work with.