Interconnected ground planes on signal/component-layer
The datasheet does not make any claim that the TTL driver is isolated from the RF side of the switch, so even if you isolated the two grounds in your layout, the chip may internally tie the grounds together.

I see two options:


Use an opto-coupler before the TTL input so that you are sure your digital circutis are isolated, then use the recommended layout.
Buy a chip, and make some measurements verify that (a) the TTL ground is not internally connected to the RF ground and (b) the chip will still operate if the two ground planes potentials drift apart a little bit. If you are sure of both, then you can modify the layout.