He's a Chartered Accountant, not even an economist. He's equivalent of someone like Rupert Lowe saying they don't need to know about trade economics because they're a businessman. Not sure he's even got a tax qualification (lots of people work in tax without the additional tax qualification, but I'd be interested to know if he has).
On MMT, good discussion here between Prof Jo Mitchell and Tim Rideout that's worth following through.
In a nutshell, MMT is a way of seeing the economy. Whether you think it's useful or not, it is up to you, but you can reach every desirable policy choice via more conventional economic models. Except those are a bit more up front about the trade offs. MMT tends to get corrupted into "government can just create money and do all this stuff now", with the "well, yeah, taxes would have to rise in time" being added only when pressed. At which point you'll probably be wondering what the point of MMT is.