Just to clarify what everyone is seeing here, the FJELL HHKB's plate seems to be Case mount. There are grooves within the inside edges of the case for the plate to rest on, while the tabs are to hold it in place.
Think of Kin's TX boards. If you have one, you'll know how solid of a typing experience it is.
At first I thought it was only held by the 6 tabs too, until LeandreN pointed out the inside edges of the case.
I don't think there will be any ping at all with the case.
If im incorrect LeandreN, please correct me!
Edit: Overall, this will be the ''must get!" of 2017. Amazing work, LeandreN!