Those are 400 shortform articles (each 150 words or less). Motivated as they are, willing to stretch their limit they should be able to do it. In time they churn out quality consistently. NaNoWriMo writers can write more than 1600 words a day. Kudos to them! Yes, we can truly learn from them. You probably should ask those writers directly how they do it. I'm sure some would be willing to guide you. Medium is a good community, isn't it? Writers help fellow writers, don't they?

Don't underestimate yourself but try it yourself. If you have the same motivation and excitement, you might just achieve it too. I wish the best for you, Ossai! Peace be with you.