It just wouldn’t make any logical sense, given the timing of her death.
Xiahou Hui died in 234 - the reason given for Sima Shi poisoning her was that she knew about his family’s plans to take power but this really doesn’t make any sense in 234, for a number of reasons.
1. Cao Rui was emperor at the time and was only 29 years old. He was superhumanly intelligent and beloved by all of his ministers. There was no reason whatsoever to assume that he wouldn’t still be around for the next 50 years. With a young, talented emperor on the throne, there was no reason for the Sima family to think it would be in any position to take power. It was only because of the power vacuum created by Cao Rui’s sudden death at the age of 34 that the opportunity arose.
2. Any plotting that Sima Yi may have been doing probably had little to do with Sima Shi. Sima Shi had nothing to do with the Imperial Court until 237, and he probably saw very little of his father for many years, since Sima Yi was usually stationed at Wan or Chang’an. While there could ahve been some contact between them, even if Sima Yi did for some reason think he’d be in a position to take power, he wasn’t in a very good position to communicate such plans with his son.
3. The Sima and Xiahou families were very close at the time, and there was every reason to assume that the Xiahou family would support any bids that the Sima family made for power. They were, after all, linked by marriage - what was good for the Sima was good for the Xiahou. So even if SIma Yi was plotting to take over and was somehow communicating such plans to Shi, it would be rather hasty of them to assume that Xiahou Hui would not support them.
4. Even long after Xiahou Hui’s death, the Sima family still treated her family well, particularly her brother Xiahou Xuan,. Xiahou Xuan was deeply in Cao Shuang’s inner circle, as deeply as the likes of He Yan or Deng Yang. Yet when Sima Yi executed all of Cao Shuang’s suppoters, he not only spared Xiahou Xuan, he promoted him to be Minister Herald, and then Minister of Ceremonies - both of which were extremely prestigious appointments. Such special treatment would seem to indicate some affection on the part of the Sima family.
5. It’s also worth noting that although Sima Shi married two other women, he never had any children with either of them, and there is no record of him taking any concubines in spite of his high status. His only children were his five daughters from Xiahou Hui - he had no children with Lady Wu or Yang Huiyu, though he and Huiyu did adopt a son (Sima Qi, Sima Zhao’s son). While there may be any number of reasons for this, it does suggest that perhaps Sima Shi loved Xiahou Hui and simply did not desire to procreate with anyone else.
6. Sima Yan made Xiahou Hui a posthumous empress. Giving her such an honor seems like a very strange thing to do if Sima Shi murdered her.
All of these points of fact cast serious doubt on the allegation that Sima Shi poisoned Xiahou Hui. It just doesn’t make any sense.
While the SGZ was written shortly after the events it describes and is generally very accurate, the Book of JIn was not compiled until 648, nearly 400 years after Sima Shi’s death and after countless records were lost in the chaos that followed the fall of Western Jin. Given this, when the information contained in it clashes so badly with other facts, I have to doubt it.
It just doesn’t make any sense.