the
I really don't like to oppose here, but Ennis don't move his lips, not a little bit, the voice is Jack's (all 3 "it's alright" are spoken by a same person), and Ennis is not an apologizing character. He didn't enter the tent to apologize. He was too vexed about everything that he wasn't thinking about the apology at all.
During FNIT he wasn't rude at all, maybe rough, but Jack gave the clear sign what he wants and he enjoyed every inch of Ennis' roughness
! Even if it was painful.
What they really said in there during SNIT is very difficult to prove, so I won't oppose to this one any more. I'll just say that I'm 99% sure about this.
Another words said there are also Jack's who says: "Come on..." just before kissing Ennis! 
Listening with the same care-and high volume-than the "I love you" of the shirts scene,one can heard Ennis saying "I'm sorry"... ^^) .You're right,Ennis is not the apologising kind,but even so,in this very moment,the same than he felt he had to sureender somehow to Jack in a physical sense,he felt also that he had to apologise before him in an emotional sense.He didn't need to say anything and the mere action of entering the tent speaks for themselves,you're right again...
But there were some "little behaviour conduct" that IMO had to be "forgiven" with words,not only with this fact;Ennis maybe wasn't rude in FNIT,but well,let's say that he wasn't too delicate either ¡

.But I guess his way of "exploding" before some sexual tension situation-very logical,on the other hand...-,hadn't been exactly the same that Jack would have wished-more foreplay,let's say again,for what Ennis wasn't prepared still...-.Nor his behaviour the morning after and when Jack was going up to the mountain to talk,one more time not exactly the same than this last hoped.I think,in sum,that-even if the simple fact of entering the tent was enough-Ennis apologise makes sense in this concrete moment.JMHO.