https://www.msn.com/en-au/health/other/scientists-discover-why-stress-helps-spread-cancer-offering-n

As far as I can tell this is in the theory category. Chronic stress isn’t good for anything, while occasional stresses can actually improve our overall health. I’m impressed by what cancer researchers work on, and the connections (sometimes obscure) they can make. But replicable results! As @GorgyS says, stress and a cancer diagnosis go hand in hand and stressing about your stress - well, anything that can reduce that stress is beneficial
until we can get clear and actionable information about what stress does and doesn’t really do.