Well, I think I can prove that it is safe!
Base on the video, and using the operator foot dimension as a measurement unit (1 foot).
The concrete is:
Width = 6ft
Length = 12ft
Height = 0.5ft
Density = 143 lb/cubic foot
The weight of the concrete will be; W = W * L * H * D
6x12x0.5 x143 = 5148Lbs = 2335Kg = 2.335 metric ton = 2.5 US ton
I really don't think we should recommend this method. Besides, I think your calculations may be a little out, it certainly doesn't look 6ft wide for a start. Granted it will still be very heavy but and may be unlikely to topple but I'd still rather do it with the proper equipment.