Trump is addressing the dilemma republicans are creating for themselves by not being logically consistent. Many republicans/conservatives questioned Obama’s eligibility using a standard they now refuse to use with Cruz. Logical inconsistencies like that makes republicans/conservatives look like idiots.
I doubt there's a lot of inconsistency. Those who questioned Obama being a natural born citizen are, by and large, the same lot who are questioning that of Cruz (and Jindal and Rubio). They believe erroneously that natural born means born on U.S. soil to parents of whom at least one is a U.S. citizen. Some believe both parents must be citizens to be considered natural born.
The objections to Obama's legitimacy were 1) those who maintained that both his parents needed to be U.S. citizens even though he was born on U.S. soil, 2) he was not born in HI or 3) a combination of 1) and 2).
I never doubted Obama's legitimacy as I hold to the Harvard Law Review's assertion that natural born is the same as citizen at birth (as opposed to naturalized).