I agree, but I don't see this as an issue, as written in the description, "might be handy... feel free to update those methods".
Could you re-add it as suggestion? (having open issues blocks this kata from being approved...)
Thanks!

You are making extra calls to getHeight then required. Would increase your time of execution.
Holding the height infomration once caluculated might be a better solution.

To have an optimized solution you do not need isPerfect inside your TreeNode class.
This just hinted towards a more complicated solution.
Check my code for the reason.