Is it true that transfecting 293T cells with shRNA plasmids results in the formation of retrovirus and that this is true for all mammalian cells with large T antigen?

No, the shRNA constructs must be made into virus using full packaging systems as the shRNA constructs themselves do not include the needed genes (gag, pol, vsv-g, etc.) as provided by the packaging systems. For pSM2 constructs you will need a retroviral 293-packaging system. For pGIPZ, pTRIPZ and pLKO.1, you will need our Trans-Lentiviral Packaging System that is useful for 2nd and 3rd generation vectors (