With the -k*[keywordspec]* argument of xgettext you can add keywords. The default keyword specifications stay enabled, though. And for Java, these keywords include getString, no matter from which class:

• For Java: GettextResource.gettext:2, GettextResource.ngettext:2,3, GettextResource.pgettext:2c,3, GettextResource.npgettext:2c,3,4, gettext, ngettext:1,2, pgettext:1c,2, npgettext:1c,2,3, getString.

To disable the default keyword specifications, the option ‘-k’ or ‘--keyword’ or ‘--keyword=’, without a keywordspec, can be used.

https://www.gnu.org/software/gettext/manual/gettext.html#Language-specific-options

So, just adding -k with an "empty" keywordspec removes the default keywords.
Considering this, the full command should be:

find src -iname "*.java" | xargs xgettext -k -ktrc -ktr -kmarktr -ktrn:1,2 -o po/keys.pot --from-code=utf-8