1. Since ComboFix is also a signature based tool, many of the signatures that it uses to compare against in order to search out and destroy malware look to Symantec like the malware that they are also trying to search out, or...
2. Symantec would really rather that they own to anti-malware market, and so attempt to frighten you away from using other tools.
By the way, the first thing that Combofix does is tell you to turn off other antivirus tools. This is not to infect your machine, but to prevent interactions that could cause either system to go haywire.