His English is just a thin veneer, so his weaknesses quickly show.
If you argue with superficial knowledge, you'll be refuted right away.
blunt sword with a tempered edge
Original literal meaning: a sword made by welding a hardened steel edge onto a softer iron body, producing a blade that looks sharp but is actually weak. Now mostly used to explain the origin of the figurative sense.
Originally, 付け焼刃 referred to a poor-quality sword made by attaching a tempered edge.