Concrete was TOO dry?
One thing is for sure: It could have benefited from better consolidation. This is usually accomplished with vibration. Vibration causes the mix to liquefy, allowing trapped air to escape and allowing the fines in the mix to make a smooth uniform surface against the form.
Professionals often use a power tool that actually vibrates; they immerse it into the wet concrete and move it about. Professionals, and DIY too, can also vibrate a post form like this externally. Do it by tapping/striking the outside of the form with something. Maybe a hammer/mallet, maybe a stick, short piece of lumber, a stake, etc. Hit it just hard enough that you see the concrete inside shake a little. Moving a stick through the concrete in a reciprocating up-and-down motion also helps.
My eye for concrete isn't quite good enough to tell whether your mix was truly too dry, was set up too much before it was placed in the form, or was practically perfect in every way but simply didn't get enough attention to consolidation.
I'd be a little concerned about whether water freezing in those voids might eventually cause the concrete to crack. Given that it's for a treehouse, and that there's a generous helping of reinforcing steel cast into the concrete, maybe it doesn't matter too much.
If you do re-pour these, you could consider placing a post base or an anchor bolt into the concrete before it sets. Sometimes that's a little more convenient than drilling holes and bolting a base to it after the concrete has hardened.