While I dont think Macron made the best call there, its not entirely accurate to say the left coalition won a majority. They won 182 out of 577 seats (31.5%) which makes them the largest minority, but not a majority.
Coalitions don’t necessarily include the largest minority. If the other parties for whatever reason find the differences too inreconsilable, then you cannot form a coalition.
I was reading an article on that on Tweakers two days ago, and I was quite confused until the last sentence saying that it was not related to the “decentralized chat protocol with a similar name”