Numpty Q about 69’ers

Readfing the review of the Singular in this months’s STW, it says it can run a 29 front with rigid fork but the frame is also designed to run normal 26 wheel with a sus fork.

What’s different then to running a rigid 29’er fork and wheel on the front of any standard hardtail to get the same effect?

Only thing I can think of is the toptube length and does it make issues with overlap, etc. But as most bikes now tend more towards the long TT / short stem is that really going to be an issue.?