I know Siri works better on Homepod, but that's probably due to the additional microphones and processing so it can more easily tell that you're speaking to it, and what you're saying.
Pullstring doesn't do voice recognition so much as they make tools which allow you to create Alexa skills and Google Home actions.
I think Apple bought Pullstring because they want Siri to be able to do more complicated things like Alexa skills and Google Home actions, and Pullstring has already spent a lot of time making those.
Sure, the Home app can turn lights on and off, but its abilities are primitive when compared to skills and actions, and I think Apple wants to add that functionality to Home.
I think voice is totally overrated. I'm not going to dictate my quarterly report or use speech to write my book. I need time to consider, to gather my thoughts, and those pauses confuse voice recognition systems. I don't know that voice navigation is so important that Apple should spend tens of millions of dollars on it, but they're pretty smart about their acquisitions so they must have some plan or they wouldn't spend the money.