They wouldn't be providing services in France, they'd be providing service in Switzerland to people from France. No different than if someone from France traveled to a bagel shop in Switzerland for a bite to eat. If France wants to make it illegal for French citizens to communicate with a foreign business in Switzerland, I don't see how it's the business's responsibility to enforce that.
If someone put a bagel shop on the Swiss/French border and sold bagels across it (using, I don’t know, a long pole or something) I guess there would be some questions.
Realistically, Switzerland and France have a relationship; treaties, etc. Quad9 has to comply with Swiss law, and they probably hope that making reasonable good-faith moves to block French users will cause Swiss courts (if it comes down to that) to see them as the more sympathetic party.
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