Active Matrix grayscale screens are, if anything, MORE readable by passive light. There is only one set of liquid crystals for the light to passthrough to hit the backlight's reflection layer.

The original Macintosh Portable had an active matrix LCD, and no backlight. It was later revised to add a backlight. Many early PC laptops likewise had no backlight.

That said, LCDs that are designed to be passive-light only tend to have a different backing layer that makes them more suited for that use. Heck, even my MacBook Pro (non-glossy screen,) is usable, if only barely, by reflected sunlight.