If they fought Howdrowf in Castle Doomfist, and he had regular ogre stats then, and they have some way to recognize him when he tracks them down, then no, he's not a solo. His stats remain nebulous until it becomes necessary to figure out what they are, but once they've been established, he doesn't miraculously get tougher when he's by himself--or at least, not so much tougher that the PCs might notice.
Of course, in that case, why the heck is Dirk sending him after the PCs all by himself? Shouldn't Dirk pick someone with a fighting chance?
I understand what you're getting at; it will seem odd if every ogre the PCs meet who's all by himself has bad-ass solo stats, but every ogre they meet as part of a gang has regular-ogre stats. And if the PCs cut some random ogre out of the pack at Castle Doomfist, I'd probably give him regular-ogre stats and let them shred him... just the way I'd let Doomfist himself shred them if they were insane enough to pick a fight in the throne room. That sort of thing adds to verisimilitude; not every fight should be precisely calibrated to the PCs' level.
But if I'm setting up a key fight by having the ogre come after the PCs, then the ogre in question is going to be a special, extra-tough ogre handpicked for the job.