Update: Yesterday I took note of the earliest video.
Today, I compared the earliest video and 11 of the yesterday's videos did get overwritten. So it's all good.
As for the KB X300 being a cheapo, yes it is. But it did teach me what to look for in better cameras. This camera came with one battery. I asked Kaiser Baas if there were spares and a charger. Yes to the spare batteries. No to the charger. That means after long rides, I have to recharge the batteries in the camera one at a time. Not only that, the camera does not have residual memory. Every battery change requires a reset if the date and time.
And then after a year of use, the casing started cracking and falling apart.
So in car-mode, it's using the power from the car accessory plug. That means I never have to reset the date and time.