Actually, Owen *is* a speedster -- in OUTSIDERS: FIVE OF A KIND he outruns the mach-force suction of a catastrophic decompression on the space-station he and Nightwing are on.
Now, whether he's a *competent* speedster is certainly open for debate.
Technically, in the DCU every speedster is connected to the Speed Force, except Zoom/Hunter Zolomon, who is actually manipulating *time* to give himself superspeed.
Thawne didn't say he was going after all speedsters -- he said he was going after Barry's family. But he killed Savitar and Lady Savitar, who were not friends/allies at all; they were *enemies* of Wally -- they didn't even exist when Barry was around. If Geoff Johns is bring in every currently living speedster, it seems a little odd that he excluded Owen Mercer.