It's all about Nokia, not any hardware limitation
You don't know this. This is your assumption, and while that is fine, we all need to acknowledge that a very high probability exists that our assumptions are wrong.
What I do know (now), is that it isn't possible to guess a display's battery drain based only on its technology (LCD vs. AMOLED) and screen content. That is a gross oversimplification.
An LCD panel's battery drain depends on many things. The number of backlights/display size, their quality/cost, brightness (which increases at 1/4th the rate of applied power), and a dozen tricks OEMs can use to reduce power usage, not all of which can be achieved with any LCD panel, like the ability to power up only some of the backlight LEDs. Anyway, it isn't intuitive. For example, the display on the L520 is quite a power hog, despite being the smallest LCD display on a Lumia device.
My point is, while you may be correct, in that Nokia is using the glance feature as a means of product differentiation (high end vs. low end) display technology is complicated enough to acknowledge the possibility that battery drain truly may be the main issue, particularly with lower cost LCD panels, as used on the L620, L520, etc