they in the sentence refers to the right brained individuals.

because a's dont do as b's do, they cannot do xyz: it is reasoning why a's cant do what b's do

"They may not notice and remember the same level of detail as their counterparts", is not a modifier (and not modifying left-brained individuals). Instead, "as left-brained individuals do" is a non-restrictive (non-critical) clause, and the sentence can be read without it.

Because right-brained individuals do not employ convergent thinking processes they may not notice and remember the same level of detail as their counterparts.

Two tips:
1. Always look for clauses offset by commas, and see if the sentence makes sense without them.
2. Modifiers always modify the word right before the comma. If there was a modifier being applied to "left-brained individuals, "left-brained individuals" would have to be placed right before comma (instead of the word "do").