"After that the 4k selected will participate in a coding session (perhaps kind of like a mini hack-a-thon) sometime during the summer, from which 1k “geniuses” will be selected."

Based on what the website and press releases say, it seems like it will be closer to a 1-month long, 15 hours per day coding session. I wouldn't call that a "mini hack-a-thon".

They use the term "piscine" (pool), which is the term already used for a similar training step at Epitech (founded by the same Nicolas Sadirac mentioned in the article), another private tech-focused school based on similar ideas of "code, only code".