Hello Doug, it is indeed true and I am not the only one to say so by any means. Talk to Stig at Lyra and he will tell you the same thing and has posted to that effect on this forum.
You are correct that a low impedance is what is needed to detune the RF circuit. Also, SUTs don't pass RFI very well at all, so they can help too. But I find that SUTs also impair detail, so I prefer to do without them if I can.
You are also correct that only a few phono sections don't need the loading (obviously ours are examples in that regard), the thing is, its easy and cheap to fix this problem but as you surmised most phono preamp manufacturers (and quite well-known at that) don't even understand what is causing the problem so they are not in a position to fix it. So the myth survives quite easily.
One note- high output MM cartridges are different in that they are very much affected by loading at audio frequencies! But oddly enough, you hardly ever hear about anyone doing it, which is really too bad since many MM can easily keep up with much more expensive MC cartridges once the loading issue is addressed.
