You can say that all you want, but that's not what the rule says. It says very plainly that a Snell 2005 helmet is required starting in 2011. If doesn't say a Snell 2005 OR a Snell 2010 helmet is required in 2011, it says only 2005.
Firstly, YES, you are correct in the rulebook quote you referenced.
Second, let's use a little common sense here. As has been previously stated, the "intent" of the rulebook is to establish MINIMUM safety requirements, that must be adhered to by all. If you want to surpass those requirements and go for additional safety measures, the more power (and intelligence) to you.
So, to make it unequivocally clear, for the 2010 racing season at Maxton, the ECTA will accept a Snell Foundation M2000 or SA2000 or later Certified helmet.
Beginning January 1, 2011, a Snell Foundation 2005
[or later] rated helmet will be required (Manufacture date not to exceed 5 years). All enclosed cockpit car and motorcycle streamliners will require an SA rated helmet.
I hope this helps ....