I can reproduce this on openindiana 148 and 151. There are two separate issues with yelp displaying the man page for zpool(1M):
First, yelp doesn't know about the section 1M man pages, thus "zpool(1M)" isn't found. The 1M man pages don't show up when you start yelp and click on home/manual pages/system administration (or on any of the other manual pages sections). They also don't show up when you do a search. I suspect this is because yelp doesn't understand the openindiana/SysV man page section numbers - I found an old opensolaris bug from 2009 on this: https://defect.opensolaris.org/bz/show_bug.cgi?id=12013
Second, when you tell yelp to open the man file directly it gives an xml parsing error. In particular:
:~$ yelp file:///usr/share/man/man1m/zpool.1m
pops up a screen that says:
XML Parsing Error: not well-formed
Location: file:///usr/share/man/man1m/zpool.1m#x-yelp-index
Line Number 1020, Column 12:
</p><p>tab(ctrl-G) box;
-----------^
(well, the error shows a symbol instead of ctrl-G, but it corresponds to ctrl-G in the zpool.1m file)
I found an old opensolaris bug from 2008 which may be the same problem: https://defect.opensolaris.org/bz/show_bug.cgi?id=5063
If I edit edit zpool.1m and remove the three ctrl-G's there, then yelp is able to display the man page, although the attributes table at the end does not display correctly, so it looks like yelp will display an xml error on any man pages with ctrl-G in them.