OK, the dimensions are fine, indeed.
I looked into the code one more time and I don't see much in the way of limits. There are some limits, but they are huge, bigger than the image in the screenshot. I also did a test export. I couldn't use a 700k x 700k, 3.3 TB image, unfortunately, but I used a 50k x 50k, ~20 GB one with the same pixel type, the test completed fine.
The only thing that I see is that the image might refuse to render, because the index is not there or is not prepared to do that. But I don't see how that could happen if you used the Merge dialog. The Info pane shows that you do have an index, too, so the first worry is ruled out. Still, if you open the image, does it render fine? Does the window ask you to update intermediate levels (by showing a red icon on the image tab)? If it does, update intermediate levels, save the MAP file (I know, a long time, but if you don't do that and something fails, you will have to update intermediate levels again), then try to repeat the export.
Finally, if nothing seems to help, two tests:
Copy and paste the image (not the table), open the properties for the image copy and set the rect to something small (eg, if it was [ 1234567, 1234567, 998765, 998765 ], set it to [1234567, 1234567, 1235000, 1235000 ], about 1000 x 1000). Open the image, observe that it renders fine, then try to export that. Does it work or does it still fail?
Try exporting the big image to TIFF. Does it at least start to export or does it fail too (give it 3-5 minutes to try to fail, then cancel)?