My understanding is that he can make janky approximations of the physical aspect of primarchs. But a true primarch is something like a demon prince, the Emperor bound some portion of warp power to the genehanced bodies he created. Cawl’s Primaris primarch knock off Alpha Primus illustrates this, he’s powerful, but also in constant pain, and unhappy with his lot. He’s a janky facsimile of the Emperor’s great work.