No you are not wrong samhy, but if you have the light turned up high during the day there may be a bit of a difference (probably 3 weeks instead of 4 weeks battery?). Generally speaking, there shouldn't be a huge difference, and depending how you use it, very minimal.
The
specs for the Glo state "One month of battery life is based on reading 30 minutes per day, turning 1 page per minute, with ComfortLight turned on OR off." (The Touch states "The Kobo Touch battery lasts up to a month a single charge."
Same is true of the Kindle Touch/Paperwhite (the specs are a bit more revealing) - for the Touch "A single charge lasts up to two months with wireless off based upon a half-hour of daily reading time." for the Paperwhite "A single charge lasts up to eight weeks, based on half an hour of reading per day with wireless off and the light setting at 10."
People have their preconceived ideas of what difference the built-in light will or won't make. I think many people will have to own one before they change their mind though.
Both the Glo and the Paperwhite have been out for a while now, and there are still plenty of people who are sceptical about them, but mostly from people who don't own one. For those who do, while they may not be perfect, the light is the one thing everyone agrees they wouldn't do without in future.