It's the opposite: It is a civilian ship that got a few more weapon mounts bolted on the hull to prey on freighters. The description mention that pretty clearly:
While degraded by cycles of bad maintenance and combat repairs, it is obvious to the keen observer that this non-standard hull wasn't modified after the production, but is in fact a distinct model. For a long time the Diable Corporation has been secretly arming pirates with these altered ships, but nowadays it won't even bother hiding that fact and sells them proper military craft directly. As such, those pirate themed hulls are somewhat a rare sight.
This however doesn't correspond with in game "stats" then. The OP pool is the same as for the standard Rime, and if it had been a distinct production model, then it should have at least 20 OP points more to cover up for 2 additional medium mounts. That though, Rime P would still be extremely weak even with those, but it would be at least able to play a role of a missile/support cruiser without getting literally double-crippled.
Being honest then, a pirate conversion after production would make more sense for how much under-powered and weak Rime P is (with or without 20 OP points, as I've tested it by adding those 20 more OP myself - Rime P is still really weak but at least usable - that's however my personal opinion). I still love it though, Rime is in general my favorite ship design in general. But I think Rime P would use at least 20 more OP to use nevertheless. The same applies to the pirate stratus more or less.
Both "P" are really unique find among pirates and it's just a bit of a let-down that they turn to be absolutely harmless. I love DA and DA's ships, and I simply think that if both types are sacrificing so much of C/F-space then they should get relevant OP to cover up for the gained mounts. Such ships with DA's weapons (one of the versions) would be lovely ships to face-off against, for sure way more of a challange than regular pirates or current versions that suffer from lack of OP.
The ships though are super rare (saw only one pirate stratus in space, saw way more of stratuses and rime's on black markets so far) so it's not much of a problem... Unless someone would like to try do a "pirate" playthrough...