Currently they are sorted numerically, checking some binary i have a original rc file for i can tell that the order inside the binary matches the actual rc file order.
In this particular case in the RC i got dialog 135 followed by 129, but when opened in editor it shows dialog 1 followed by 128. Indeed checking the strings in binary however it is dialog 135 followed by 129.
Edit:
Looking into this more you use the winapis to query the resources which parse the resource table
So this is the resource table order.
I guess to restore the binary and thus actual source order would need to parse IMAGE_RESOURCE_DATA_ENTRY entries in the resource table after the fact, reading the OffsetToData and re-sorting the resources by that.