I'm violently agreeing with you.Understood, there is a gaggle of information in the video that challenges the validity of it as ever being a purity law. This man suggests it was a law predicated on taxation.
Amazing how that works! From the histories of brewing I've read, taxation as much as the available ingredients led to development of what we call beer styles.The other tax law that's strangely become a 'tradition' is the turbid mashing in Belgium. Turbid mashing was developed to reduce tax as they were taxed on mash tun size. There are plenty of people that think it's fundamental the beer style, not a work around to reduce tax. But then they've brewed the style far more than me.