Perhaps you’re right, and perhaps Mr. Costella is right ... but just the tone of Mr. Costella’s rant makes me hope you’re both wrong. ;-)
I was just remarking on that one aspect — Costella’s concern may turn out to be misplaced. I haven’t formed any opinion of the relativistic physics — not quite sophisticated enough to do that, at least not without a solid weekend of cramming which I’m not going to do — but I would say the viability is unlikely for any useful thruster within our lifetimes even if the claims are true. Costella’s most useful contribution from my point of view may be his observation that relativity doesn’t impact the conservation of momentum IIRC.