Somehow texting someone is ignoring them. Big brain logic there...

/u/encoure didn't say OP was ignoring his girlfriend—he said OP was ignoring the problem. Because OP squandered the perfect opportunity to explain to his girlfriend why he was upset and how her choices and actions made him feel. Instead, he just kinda went ¯\_(ツ)_/¯

These people have a really skewed mentality on what childish is.

Here's the point where OP's behavior departed from healthy boundary-setting and turned childish (emphasis added):

At about 12:45 I got a string of messages asking where I was and what's going on. I just played dumb and said "Wait, you're ready? I thought you were still in bed or something??"

That's why some of the comments are excoriating OP for playing games and being childish.

The adult thing to do would have been to respond to girlfriend by saying something like, "You didn't answer any of my texts this morning and I felt really frustrated. Since you didn't use your words, I didn't know if you wanted to go, so I went without you. If you want to catch an Uber or something, I'll be here until $WHENEVER. Otherwise, we can talk about this when I get home because right now I'm enjoying the party."

By responding to passive-aggressiveness with passive-aggressiveness, OP is doing some of the same things that he says his girlfriend is frustrating him with: he's not explaining how he feels or why he's upset (or even that he's upset at all), and he's not giving her feedback that she can act on to improve herself, if she's interested in improving.

Granted it was spiteful, but the dude didn't put fucking gum in her hair or throw a tantrum.

I guess if you're sticking to the literal definition of "childish," OP wasn't "childish," but spite is just flat-out not a mature response. If you're in a relationship where either party employs spite as a regular problem solving tool, you're not in a good relationship.