even with a cog belt you need the engine solid mounted - I have my dry sump pump attached directly to the frame in my race car and the engine solid mounted, but also have the alternator with a simple V-belt mounted.
Agreed that a cog belt runs pretty loose (I slip my 225mm belt on and off by hand to prime the engine with a drill) and serp belts need tension, but if it is solid mounted it will work no problem (there will be enough motion in the tensioner if the belt length is chosen wisely), but in no way would I soft mount an engine with frame mounted accessories, regardless of belt system.
Can sorta see the setup in this pic, at least of the alternator. It is pretty common to chassis mount accessories to make removal and so forth easier and whenever real estate is at a premium. See it all the time in off-road vehicles. Tim has clearly built a ton of sweet cars so nothing new to him
Always amazed with this build!