They do time the race to avoid trains, but in this case the race was held up for 10 minutes at the start.
Or at least that's what they say. However, 4 hours into the race the three ETAs (for different grades of "fast") spread over 24 minutes, and they were running faster than the fastest one by 5 minutes. The train was due 5 minutes after the earliest ETA, which is hardly planning to miss it. There appears to be no other train for an hour before or after it.
That section of line would not have been high speed. It's off the main TGV▸ network. There are no level crossings on the TGV main lines.
Indeed, it's the stretch from Valenciennes to Douai, about 35 km and timetabled to take 36 minutes. The trains go on to Arras and then pick up the
LGV▸ to Paris-Nord.