Yes, Jup.....
In Fischer's chart I looked at both Saturn (always my first choice, because the traditional ruler) and Uranus as lord of Aquarius.
I use a flexible system of rulerships. I look to the chart to tell me whether the traditional or the modern rulership is to be preferred. In case of doubt I stick to the traditional. What I try to do is to determine which of the two potential rulers is more powerful in the chart in hand; I use that planet, if there is a notable difference in power, as ruler. If they are about equal, or ambiguous, I hold to the traditional ruler.
In Bobby's chart Uranus jumps out because:
1. He is involved in the grand trine, which defines the very structure of the whole chart. The triplet of Mars-Uranus-Neptune are in partile trine to each other, and the chart is built around this trine. It is the outstanding feature of the map, and Uranus is not only participant but exceptionally active and powerful.
2. Comparatively, Saturn is weakly aspected and moves away from Uranus. Saturn does not pause or return by progression. They have had their final contact, and Saturn left his "gifts" with Uranus. In my perspective, Saturn is not ignored, but is subordinated to Uranus. He is just not as active, powerful, or involved as Uranus is.
That is why I use Uranus here. Regarding the square to Mercury, it is a "cross-sign" square in that the two planets don't share Quality, as is normal, but Element. This square itself is another factor that led me to give more weight to Uranus as lord of Aquarius in this chart.
The revolutionary nature -- rather than conservative -- of Fischer's chess game and also attiitude toward the "value" of chess tournaments, while not a stand-alone confirmation, does tend to substantiate Uranus, through the life. Add to that the exceptional IQ, which is certainly something indicated by the square (but not by Saturn, if used), and there is further confirmation of Uranus' rulership. I think we should always check our judgments against the life.
But your point is valid; I agree. But due to the way I handle rulerships (on a chart by chart basis) I selected Uranus.