How so? Home Kit will have the ability to control multiple devices. Apple's own example illustrates this. You tell Siri "Time for bed" and it turns off the lights, locks the doors, turns down the thermostat, and closes the garage door. You seem to be implying that Home Kit will only be able to do that if the devices that accomplish these things are all made by the same manufacturer.
No, Home Kit will be able to bridge the gap between different devices. The short coming I'm talking about is the fact that the iDevice always initiates the actions. Home Kit will not accept events broadcast by devices that have registered with it, and then trigger actions on other devices as a result. The only way to trigger Home Kit actions is either via Siri, an app running in the foreground (background apps can't do anything), or through a scheduled event (think cron job type event).