The idea behind using a cordless drill and hole saw, etc is that we are using low-speed, comparatively common and easy to use tools, keeping it safe-ish for the average 'Not Excessively Handy' level of skill, which I think represents the majority of people who have been building these things. Certainly the people who turned up at the original workshop and have more recently been asking for assistance seem to fall into this category.
Angle grinders and bench grinders, turning at 10,000RPM or more can be very dangerous in the hands of someone who doesn't possess the proper skills - particularly when you are working on very small pieces of material like a 50mm disc. I do not believe that the sort of people who own and are comfortable using such tools would be seeking assistance to construct one of these kegs.
The specified tool set has been more than sufficient for the two dozen or more keg sets I've built. You just need to take it slowly and carefully and apply the proper amount of stupidity.