The SX won't have 13gb for games. It will have either 16+4/8 or 16+something else. It won't limit it to 13gb, it would be suicide, even more so against a higher memory PS5. The speeds are up for debate, yes, but I highly, highly doubt they would be this short sighted but in the ball with everything else, even more so when this is the exact same misstep that happened last gen (banking on Sony only having 4gb, switched to 8gb).
In fact o wouldn't be surprised to see either console bump that 16gb up even. Even last gen, 2 years ago, 16gb for games was pushing it. I see no reason why a machine that has to last another 6-8 years would be stuck with an such a low figure, when everything else is top shelf. Even more so when you think about memory being so important at such high resolutions (both screen and asset).