There's more info the the HOWTO.txt file supplied with src2pkg, but here's a couple of points from it:
One feature of the libsentry library allows us to do something similar to
using DESTDIR -even when the sources don't support DESTDIR at all. Ths method
is called the 'JAIL root' method. It uses the libsentry library to do the same
thing as DESTDIR by intercepting the normal installation commands and substituting
an alternate path before running the command in the alternate subdirectory.
The 'DESTDIR' and 'JAIL' methods can both be used while running src2pkg as a
Your selected method (DESTDIR) could not be used, so src2pkg is trying to build the package from a "jail" in a different location.