It will work from the normal stereo power supply - it just a recommendation
This is quite right. It's to do with current draw through wires on older cars - modern cars ae fitted with reasonable guage wires that will handle the current draw of high power head units. Older cars (that utilise very thin guage wire) will struggle.
If you are fitting to a mk4 for example, it's unlikely this would be needed.
Take a look at the loom in your motor and make a sensible decision based on that.