Even if Ryza is no-where near the home-base, it's plausible enough that for some (possibly long-forgotten) reason, there is a link between Ryza and the Chapter in question.
Perhaps they did not originally come from the area that they now control, hailing from near Ryza originally, and they were originally supplied by the Ryzan forges; and that connection has never been lost. They are Templars... perhaps they went on the "Centaurus Crusade" or something?
Alternatively, perhaps they acquired their connection to Ryza after the Chapter was established. Maybe they sent a company or two to the defence of Cadia that were then diverted to an emergency threatening Ryza instead, and in gratitude the Mechanicum tithe the chapter part of Ryza's production...
Or, as dbt very cunningly suggests, perhaps there is some stable warp-transit between the vicinity of Ryza and the area you want, making travel there relatively easy.
Absence of evidence is not evidence of absence indeed.
:looking for the missing piece cyclops:
"Well it's Forty-one Thousand Nine Hundred Sixty-nine OK -
Gotta war across the Milky Way - "
Iggius Popiscus and the Stoogii, "41,969"