It seems summary feature doesn't get imported correctly from XMind. (Summary is when you can group 1 or more adjacent nodes together to create a summary node).
Hi @kc_denton and welcome to the Community!
I've just checked this out for you and it turns out that this is actually intended behaviour since MindMeister doesn't have a summary feature. All your other nodes should be imported normally just without the extra summary as you can see in the image below.
I hope this helps explain things. Let me know if I can help with anything else.