Yes this is accurate. It should be in online help, maybe not. A lot of documentation has been moved around in the past couple years and this is not a common request, although it is a very useful feature. I will look into making sure we have a link for this.

So if I replace D1, rebuild, replace D2, rebuild, replace D3, rebuild, replace D4, rebuilt, and then Resize Volume, all the data is completely valid?  

Just to be clear. Approaching max volume of my current setup, should be a common situation, and it should be published into an article.  I've not found one.  Again, this is essential stuff.  

Your terminology is confusing, but let me try to parse this:

Rebuilding a volume is easy. Just replace the disk with a larger or same size one and it will rebuild. But even with a much larger disk the volume is the same size.

If you have 4 drives, you could replace disk1 with a 10TB, rebuild, then replace disk2 with 10TB, etc. When all 4 are replaced, you can "resize volume" to get a 30TB total volume size.

You can do this without backup/restore, yes. But you do not get a larger volume, until all 4 are replaced, and THEN you can use "resize volume to make it larger.

A volume is comprised of all EQUAL size partitions. Putting a 10TB will result in a 2TB partition on that disk.

Only when all your disks have 10TB drives, can you resize your volume.

the smallest disk limits the size of any RAID volume.

This "feature" of variable partition sizes is what essentially caused Drobo to go bankrupt.