I'd say it's bad info. If that were the case, then if people magnetise their water before brewing should not be doing that, passing water through magnets prior to brewing. No, I believe it's bad info. {opinion}
I've been using my home made magnetic stirrer for near on 10 years and would not be without it for any volume over 300ml or an 8 ounce glass.
I believe Esstee summed up the reasoning fairly well.