"...accusation being that it uses this closed code
to promote its own products ahead of rivals.“
Search only matters in how it might be a tool that can be abused to gain influence in other, not-search markets.Being a monopoly is usually legal. Monopoly status simply means new laws apply relating to how that power is used. Google is patently a search monopoly right now, so France is well within their right to accuse them of abusing that power to take over other markets. Some sort of trial will determine if those charges are true or not.
What is obvious - regardless of the outcome or politics surrounding France's legal action - is that the exact nature of how Google's search algorithms work is the exact evidence needed to properly judge how Google's search service (via the algorithm it relies upon) is unfairly interfering with the markets that Google may also be participating in.