For Little League, I believe the statement is true.
But at the higher levels, I don't. For instance, there is the scenario wjere a coach pulls a position play off the field to pitch because he doesn't hit well and/or is weak defensively. I have seen position players "turned into" pitchers because their arm was good but they were struggling elsewhere in their game.
At the college level, tho there are some players who pitch and play defensively elsewhere, we have found that most "two way players" discover ... as our son's college coach said ... that they are doing two "full time jobs" and eventually, one gives way to the other because the player finds it difficult to juggle both.