Which brake first?

Sticker on my ebike seems to point to the front wheel/brake.
Really 4 pages of this stupid roadie bullshit.
Common sense and experience should tell you that applying both brakes is the best practice and that knowing the braking characteristics of your bike is essential.
If you're the type that needs to be told such a simple concept by an expert authority... Then your spandex is too tight and your balls have been squeezed into your brain which are obviously up your ass and a trip over the handlebars might be just what you need to dislodge them. 🙃
On my first ebike, I applied both brakes together until the day I grabbed the back brake to turn the bike up on its back wheel. The pads had worn away because the caliper had come from the factory misaligned. At the same time, the cable housing had stretched, which is normal for a mechanical rear brake. I hadn't squeezed either brake hard enough to realize that the rear lever had become spongy.

Since then, I like to check the brakes separately in the course of a stop. In that connection, I found that the mechanical brakes on my Radmission would consistently stop me from 20 mph in 20 feet, and I could stop slightly shorter with my front brake alone. The tire gripped well enough that I could have squeezed the lever harder, but I was at the point where that would have sent the bike end over end. With almost no weight on the back wheel, that right lever was just a distraction.

I've never been ejected over handlebars, but I went end over end 62 years ago. I finally had the money to buy a bike of my own. I chose an old Jackson and paid twice what it had cost new. The fenders were perforated with rust. On my way home I passed a homicidal young man. When he suddenly hit the back of my front wheel with his basketball, I knew what he intended. The impact would deflect the front wheel out from under me and I'd hit the pavement on my face. That bike offered such stable seating geometry that not even a 22oz ball thrown at 45 feet a second could deflect the front wheel. However, the fender crumpled against the top of the fork, locking the wheel. As the bike went end over end, I flipped over the bars and landed on my feet.