Responding to low-key insult from lecturer
Sincerely? Let it go. The tone is also not "low-key insulting" (that means a personal disparaging attack on you), it is dismissive/snotty/irritated (do not bother me with this stuff).
The thing is that you are reading a lot (IMHO too much) into this. I'm not saying that you are wrong, but it sounds that you are in danger of spending precious lifetime for collecting perceived slights. You have already used approx. 1 hour of your lifetime to ask this question, and probably several hours of suppressed anger pondering his answer.
Really, the easiest way is to think "Perhaps he has eaten something bad today/has relationship problems", or whatever. Remember: once is chance, twice is coincidence, thrice is a pattern. If it happens three times, then it is time to act.
It sounds like the instructor was about 10 minutes late to class, leading to the entire class being confused about whether or not to go into the classroom?
That's somewhat embarrassing for the instructor. By being late and forcing a bunch of students to wait, in addition to apparently having some confusion about location, they dropped the ball. And especially if they're worried about getting tenure, they may've been concerned about coming off as unprofessional in the eyes of the students who'll be writing their teacher evaluation at the end of the semester.
In short, their response sounds mostly defensive to my ear. They were basically trying to say,
I was just 10 minutes late, and I don't think that there should've been confusion about the classroom location. Please don't dock me on the evaluation later!
The appropriate response might be to be supportive.
Directly emailing them back would probably be the wrong way to go, as it'd seem to make the issue larger. Rather, what's needed is for everyone to move on.
If they're a more secure type, then basically not bringing it up again's probably enough, since they're over it.
If they're a less secure type, then a nice-but-subtle gesture can clarify the issue. This might mean asking a good question in class that they'd enjoy answering (on an interesting point; not a gotcha-question, but also not trivializing), followed by a quick-but-sincere "thank you" after they answer.
And, that basically covers their side of it. Then there's your side of it.
The way I see it, you've got two approaches:
@ThorstenS.'s approach, for the reasons they'd stated.
Emperor Palpatine would recommend dwelling on it and internalizing such injustices into an inner ball of fury, if you'd like to transition to the Dark Side.
Now @ThorstenS.'s position has its advantages. Stuff like having a happy life and healthy mind set can be enjoyable. But then again, anger leads to hate, hate leads to... something... and, in the end, there's cookies and possibly lightning hands. So, ya know, personal preference there.
I concur that you're wound too tightly.
But if you cannot rest or sleep without responding, then use self-deprecating humor, e.g. "I was so confused [about the matter] that I cannot remember it today! :-)"
A note like this essentially forgives the lecturer for being late, and communicates that you hold no grudge. It also communicates that you are not a powder-keg ready to explode at the slightest offense... any other kind of response would do the opposite.
The smiley face is important to communicate that you are not taking this seriously. (Even though you are.) Several years ago the NY Times recommeded using emoticons in email to communicate tone, because that can be otherwise hard to do in a short note.
Bottom line: if you transform the incident to something mildly humorous, the lecturer will think of you in a positive light. You don't have to suck up, but in life one has to pick one's battles. When battle is not joined, it's best to leave the other person in a good frame of mind.