>>For a long-distance "touring" bike 21" would be too small. You want something near 60cm for a road bike, and 21"=53.3cm Might be a good size for off-roading or just cruising around town, but not likely for long rides.

NO
The 820 style of bike was designed to be used with more extended seatpost than a typical road bike. Size the bike according to its length. If you have a comfortable reach to the bars, then dont worry if you have more seatpost showing. As long as the post is supported to it's max extension mark it will work.
Ive hired 820s and they seem to be robust and not too sporty for touring.