The letter came at a period when Serena was trying to recover from a knee injury. Serena herself described the criticism as “unfair.” She dabbled in fashion, although not to the degree Venus did with her company, did some acting, dated some celebrities. Took off some minor tournaments. Came back and won an Australian during which Evert repeatedly referred to her as fat.
Serena almost died of a pulmonary embolism at one point and perhaps would have if someone employed by Venus had not recognized her symptoms. She also had many dangerous pregnancy complications. And took a while to recover from slicing her foot on a broken glass in a restaurant. Not sure what Chrissie expected her to do about such things, twiddle her thumbs during recovery?
I think exploring these other interests is exactly why she and Venus still love tennis decades later. They did not burn out on tennis, but did other things with their lives. But the entire vibe of her calling out Serena was that she was lazy and wasting her great physical abilities.