Corbyn's not been in government so he doesn't have anything to defend. May can be challenged on her record in office, which, as with most politicians of any party, is not exactly a string of glittering succeses. Hence the need to be defensive and evasive.
Were Corbyn to become PM he would then have to be as defensive and evasive when interviewed as May is now.
Of course he could play a blinder as PM and thus have nothing to be defensive and evasive about but that standard of performance would be unprecedented in British political history.