It does look like depth to front of the lawn/rear of patio is at least 4" so as long as the ground is slightly sloped to attain that then it fine as it does look like the grass area slopes from rear retaining wall to patio based on the gravel layer. And tbh you could easily do enough to give 6" fine with a slightly steeper slope that honestly wont be noticeable enough. It appears the drop only needs to be around 75mm over the total length to achieve that.
But the bigger issue as noted is the depth they could get to have it tie back to the original drainage and also if they have wrapped in membrane so they don't fill.
But the bigger issue as noted is the depth they could get to have it tie back to the original drainage and also if they have wrapped in membrane so they don't fill.