Interesting; there are two tables: one named Frequency specification for U1281A / U1282A at page 14 which states 9.9999MHz and another named Frequency counter specification for U1282A at page 16 which states up to 99.999MHz with a "divide-by-100" and up to 1.8VP spec. This last one works on the MHz range selection.
The manual you linked is clear about which is the Frequency Counter. I assume that divide statement is just showing that the resolution has changed, not that the meter requires some sort of external clock divider. If they did require such a device, they make no reference to it in the accessories or show it's use in any of the manuals that I found. I also can't imagine anyone spec'ing their 1Hz counter to 1THz with unknown accuracy, provided to attached the 1THz divider. Maybe the bottom of the bowl companies.
I wasn't sure which band you referred when mentioned the 100MHz, so I had to check and found the second table of the MHz range. My first message was clearly stating the first table that goes up to 9.9999MHz, which may correspond to the V
AC range. Regarding the divider, it was just to highlight what the table meant, not implying you were doing something wrong by not using any external device/accessory - I too can infer they mean a reduction in resolution.
Before we move forward, I need to clarify that I am no fanboy of Keysight or any other brand, just like or dislike each product of any brand on its own merits. The videos on my channel will evidence this.
If you can, try to see how high your meter can actually read frequency using the frequency counter function.
I need to find where I put my RF generator, but I got a very accurate measurement using my 30MHz generator set to 1.0V
PP on the MHz function.