The argument says that because candidates must prove themselves as the most potent standard bearer for their respective parties in the primary election (to even get a shot at the general election), it becomes impossible for moderate candidates from either major party to survive to the general election. Is this an innate feature of the party primary system that exists throughout the democratic world in party primaries, or something specific to the United States and its electorate?
This is impossible to answer in general objectively (you didn't provide any metrics for what constitutes moderation), but anecdotally, the answer is a clear "not necessarily".
Two last presidential candidates from Republican Party (McCain in 2008 and Romney in 2012) were the most moderate of the field when you discount people who genuinely don't represent the party at all (e.g. Huntsman's positions are more Democrat than Republican; and Gary Johnson is a libertarian odd-duck who as a libertarian doesn't have support from ANY parry :) :(
Having said that, there is a definite possibility of more extreme candidates to win in primaries, which is exactly why DNC went to the pain of introducing super-delegates in their convention after McGovern debacle.
Rationale For Super-Delegates (from about.com)
The Democratic Party established this system in part in response to the nomination of George McGovern in 1972. McGovern took only one state and had only 37.5 percent of the popular vote.