Actually, it couldn't be accurately said that way; that Soul Forging Cultivators are the bane of Body Tempering Cultivators.
It should be more like: A cultivator with a Seventh Stage Soul Forging Cultivation is able to suppress even an Eighth Stage Body Tempering Cultivator with a weaker Soul Forging Cultivation. However, that couldn't be said the same about a Seventh Stage Soul Forging Cultivation who faces a Ninth Stage Body Tempering Cultivator with a weak Soul Forging Cultivation.
After all, the difference in their cultivation stage was too vast!
In fact, Body Tempering Cultivators could also one-shot a higher-stage Soul Forging Cultivators! However, that is only if their opponents are pure soul forging cultivators who had weak bodies.