You'd need holes drilled through then. How are the areas going to be changed for your patio/hot tub base then? Or are the gravelled edges going to be wide enough that the men won't need to go near the retaining wall?
It sounds like he's installing a freestanding hot tub on a concrete base, and a patio. Either way he won't regret avoiding doing the work in wet weather and/or putting down some plywood boards to protect the lawn during the works!
I've been fairly lucky with all the building work done in various houses I've had, and particularly that recent extension. Either side of the month the men were here was our usual rain/snow/ice and wind
I live in west central Scotland - not where that photo is...
Their guidance is that it can be placed directly onto grass, BUT, if your grass is uneven or has drainage problems (check and check) to dig up the turf and lay 70mm of Type 1 MOT (I'll be laying 150mm), sand and a membrane. The pad is then put onto this and is then filled with pea gravel. I'll be doing this myself, as it's pretty simple. I'll be hiring a compactor to make sure it's done right. I spoke with a landscaper is naturally I was a little dubious and he said they're successfully used for things much heavier than hot tubs.