"In 'even tempered' tuning, notes are tuned relative to the root of a scale. So an instrument set up for, say, the key of C might sound acceptable in G, but downright awful in E or B. To play in those keys, you'd have to retune from scratch."
Although this statement is largely correct He’s clearly very confused about what it relates to. He seems to think that Just Intonation, where tuning in one key produces dissonances in others, is equal temperament!
Or "even temperament", whatever that may be...