Wow. Weren't they just angling for an IPO? I wonder how much of this was insured and what the impact is to their overall operations.
If they have a good insurance I'm confident this will have little impact on their operations, I really hope they do. I host a few components on OVH/SoYouStart dedicated servers, luckily not mission critical, but still had rather good experience with them, especially in terms of price to performance.
The timing is so bad that it becomes almost suspicious. When is the best time to sabotage your competitor? When they are the most visible.
Honest question: which publicity damage?
A fire in a datacenter is very much part of the things you should expect to see happen when you operate a large number of datacenters and will obviously cause some disruption to your customers hosting physical servers there.
Provided the disruption doesn't significantly extend to their cloud customers and doesn't affect people paying for guaranteed availability (which it shouldn't - OVH operates datacenters throughout the world), this seems to me to be an unfortunate incident but not a business threatening one.