That's misinformation. Unless you do a lossless reencode (which will balloon the filesize), any reencoding will result in a loss of quality. You can get significant size reductions with little or no quality loss that is perceptible while in motion, but to say that you can reencode video without degrading quality is false. AV1 in particular, is notorious for losing fine details, film grain, and doing generally poorly with dark scenes.
I'm not saying no one should reencode their videos -- space is finite and storage prices have climbed an absurd amount over the past year -- but the bottom line is, if you want to preserve the work at full quality, you have to keep it in whatever format the creator delivered.
