A source for that quote would be useful DG - can't see it in the manual or Knowledgebase.
It is of course completely logical that multitimbral will increase the resource load. The only relevant question is whether or not that's an issue in practice. So for for me it has not been, using Kontakt, Omnisphere, Stylus on my i7 4930 @256 - all safely within limits. Your asioguard vs soundcard settings will depend on a large number of factors. But key is assessing what proportion of your overall CPU load you'll need to access live at any one time. If you have, say, 30 multitimbral instruments, logically you could reduce the realtime load to only 1/30th of what it was using asioguard (all other things being equal). However, if you're on a modest system where you were struggling with only one multimbral instance, asioguard won't help you at all if you keep it that way - you'd be much better off spreading the load among monotimbral instruments as your quote suggests. Most pro users will have powerful rigs and quality soundcards though, for them this advice should be fairly moot - hence perhaps why they are talking it up on the main what's new page on their website, specifically mentioning multi-timbral compatiblity. Would be very strange to talk it up there and later say you shouldn't really use it.
Anyway, the only real issue so far for me has been VE Pro 5. Incidentally, I did hear that VE Pro 4 behaves much better with asioguard. That would be interesting if so.