They are wrong, though.
They state that Kanan 12 is Grand Inquisitor’s first appearance (5/2016). He showed up on Rebels Magazine #3 (4/2015), though:
KEy Collector also states that Darth Vader #6 (2017) is the first appearance of Fifth Brother and Seventh Sister, but Rebels Magazine #16 precedes it (4/2016):
They appear on 7 pages of issue 16.