The logic inputs of every Agilent/Keysight MSO that I've encountered, are single ended (with all the logic probe ground pins commoned within the pod). So, would there be any significant benefit from using more than four grounding pins?
On the MSOX, the grounds don't meet until the front-panel connector, so you have approx 1m of "floating" ground inductance on an ungrounded probe.
On a MSO-X3104A, the logic grounds are commoned in the little box that the flying leads plug into, before the long ribbon going to the scope. I was able to confirm this with some 4-wire ohms measurements. So, much less than 1m.
The 3104T probes are obviously a different and updated design. The old design is still shown in the manual, but it would surprise me if they left the ground that long per signal in the new design.
Some of the literature shows the new probes in 4 groups of 4. Maybe you're meant to use one of those ground extenders on one ground in each group, and then the ground is commoned within each group. That's why they supplied 4 extenders. Just a guess looking at the pictures.