The Zynq is the SoC that controls the U/I and all the arbitraries handling and is located on the SIMM-style CPU module under the little heat sink. The Kintex-7 is the FPGA used as the sampling engine, responsible for the data "assembly" / transfer to the DAC. Unfortunately, Siglent "economized" the dedicated sample RAM at the Kintex so the SoC has to do all the arbitraries handling, but apparently, it does the job as per spec.
I just did some testing myself and found that the external modulation input is sampled approx. every 1.8µs, and some low-pass filtering is also taking place. I attached a screenshot of a 10MHz square wave, amplitude modulated by a 100kHz square wave via the external input. An exact evaluation of the resolution of the modulation input is difficult due to the noise, but I got the impression that at increments of 3...4mV (tested with a DC "modulation" signal close to the cut-off point at -5.8xx V), the modulated output amplitude changes more than "in between", so this would make a resolution of 12 bit at least plausible. For all practical applications (checked with FM/frequency jitter), the modulation input can be considered to act "analog-like" with no noticeable increments due to the noise (S/N approx 35dB).
It's a shame the SDG6000X hasn't got more connectivity, a separate sync output would make life so much easier when trying to sync on the signal buried in the noise close to the cut-off point... But many other manufacturers also don't provide this luxury

and one has to dig a lot deeper into the wallet to find a current AWG with these features.