There is no conclusive evidence for or against amphetamine affecting creativity.
Martha Farah's comments are liklely based on a small study (linked here) she and colleagues ran in 18 participants. The tasks were brief puzzles (rather than longer meaningful tasks one might expect to be sensitive to drugs that enable prolonged focus). Results were fairly inconclusive. Their conclusion begins with:
Does cognitive enhancement with Adderall impair creativity? In this
preliminary exploration of the issue, using four different tests of
creative ability in healthy young adults, we found no evidence of a
general impairment. On the contrary, Adderall enhanced performance on one test of convergent creative thought. For this test, the Embedded Figures Test, there was also a trend toward disproportionate enhancement of the lower-performing participants. For another test of convergent creative thought, the Remote Associates Test, Adderall also affected performance, although the direction of the effect depended on the creativity of the participant; the drug enhanced creativity for the lower-performing participants and impaired it for the higher-performing participants. This pattern has been noted in other studies of stimulants and cognition and is not unique to creative thought.
I would caution that when researchers find different patterns in subsets of the data that differ from the larger group, we should regard them as very preliminary (possible fishing around for results), especially if they didn't state they intended to do that sort of subset analysis from the beginning.
Aderall is indeed made of different amphetamine salts, but when amphetamine dissolves in the body, the amphetamine ion becomes separated from the salt ions. As a result, there is no real difference in the biological effects of different amphetamine salts.
Perhaps the most famous example of a high performing individual who used amphetamines and was extremely creative is mathematician Paul Erdos, who continued to coauthor papers in mathematical journals for over a decade after his death. We don't know if amphetamine made him more or less creative, but we do know that he believed they aided his productivity.