Anderson's record in those 5 matches:
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Broad's record in those 5 matches:
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Now, obviously they were bowling to a different lineup which makes a 1-to-1 comparison unfair. However, if Steyn takes basically as many wickets as those two put together, at a significantly better average and strike rate, then there is something about the batting conditions that you're probably understating. Broad and Anderson are two of the deadliest bowlers in English conditions of the last 50 years and their struggles in those series were orders of magnitudes worse than Steyn's.
Steyn for sure had some difficulty controlling the duke's swing at times, but to use this dubious sample alone as some kind of proof that he's not good enough to be top 5 is a flimsy point. You can use the fact that he had a higher Er against him if you want, but this one, nah, it's dumb.