No matter what position they catch in, they have such strong natural throwing arms that they can make a good throw.
Literal
They [topic-は] any position [with-で] caught [even-ても] natural-arm-strength [subject-が] strong [because-から] good return-throw [subject-が] can-do.
どんな体勢で捕っても ('no matter what position they catch in') uses どんな~ても for 'regardless of.' 地肩 (じがた, 'natural arm strength') is baseball-specific vocabulary — the raw throwing power someone is born with, before technique. いい返球 ('good return throw') is also sports terminology. The sentence is baseball commentary about a player's natural gifts.