He made a huge mistake that caused you a lot of trouble and got to learn of this at the exact moment you read him the riot act (validly of course, this is not a blaming statement). So he gets to feel guilty and ashamed of causing a huge problem for someone he cares about, as well as pretty scared and tense now that the person he cares about is currently chewing his ass out and emphasizing the first part directly. In that situation, I could see myself getting a little overwhelmed by everything and trying to downplay it out of reflex, given that's about the only option I'm presented with to do something about the situation.

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I wouldn't read too much into it for the long-term, particularly if he's generally pretty considerate or apologetic otherwise. It's shitty all around and I wouldn't expect someone to want to just say "yes, you're right, I am a piece of shit and everything is awful now" over and over. You might consider revisiting once you've had some time to get distance from it (and lay down some ground rules about how to avoid this in the future, e.g. "You will only text me to confirm things you can actually confirm"; "when I need something from you, please make it a priority"). If this is a pattern, however (e.g. failing to come through on big things, not being seriously apologetic about big mistakes), none of this applies and there might need to be a serious talk.