Yeah, I agree. Isaacs returning to play prime-Lorca hardly compares to Patrick Stewart returning to play Picard (or the other TNG characters), or say bringing Shatner back to play Kirk in the next Star Trek film.
Lorca isn't a legendary character we've seen for years and grown to love, Hell, we don't even know what prime-Lorca is truly like. He'd be an almost totally new character.
And yeah, they wouldn't need any 'back from the dead' sci-fi explanation on why he is back. We didn't ever see prime-Lorca die on the series, you have a novel that hints he's still alive. All you have is speculation from an admiral who's credibility on such matters is questionable at best. He escapes from his Mirror-Universe prison and finds his way back to our universe.
universe crossings pertinent to this: as far as I can tell. Not bringing up the ds9 ones or TNG's own tangental universe with Enterprise C. dates are not definitive but i believe close.
Prime Universe/Mirror Universe: 2256: Buran incident results in Prime and Mirror Lorca's to replace each other in their respective universes. Fate of Prime Lorca is unknown.
Prime Universe: 2257: Starfleet becomes aware that Defiant is going to be lost some time around 2268.
Kelvin Universe: sometime around 2256: Montgomery Scott begins transwarp beaming experiments, losing Admiral Archer's prized beagle. Possible universe incursion?
Prime Universe/Mirror Universe: 2257: Prime and Mirror USS Discovery replace each other in their respective universes. ISS Discovery is destroyed with all hands.
Mirror Universe 2257: Loss of the Charon.
Mirror Univrese 2257: Murder of Doctor Culber, a prime Universe individual within the Mirror Universe
Kelvin Universe 2258: multiple uses of red matter: destruction of Vulcan by artificially induced black hole, destruction of Narada by artificially induced black hole
Prime Universe 2258: Doctor Culber is rescued and returned to life from the spore network, possibly the strangest crossing yet.
Kelvin Universe 2258/Prime Universe 2387: The Narada crew and Ambassador Spock cross into/or create the tangental Kelvinverse by the events caused by the attack on the USS Kelvin. As previous manipulations of the past have not been shown to create such a wide-sweeping new universe, something special occured with this moment.
2256-2258 seems to be a very active time for hopping across the three main star trek universes. And those hops aren't always static in time. Discovery took a small leap forward when it returned to the Prime Universe. If Prime Lorca were go also find a way out, he might also be moving forward in time. He might also just be driven as a starfleet officer to do something about the doomed crew of NCC-1764 in a few more years, since so far no one has seemed to show a lot of concern about them.