You are misreading the article, that cost overrun is from a previous nuke project (supplied by Areva, started in 2005 and not yet finished).

Regarding what the price guarantees are worth (and it's not an estimate, it's in contract), there are some interesting scenarios for sure if they can't hold up their promises. For example the joint company is being bankrolled with a large slice from Rosatom, one wonders what happens to their capital in case of a bankruptcy!