Porsche stated it did not make financial sense to offer the limited edition 991 GT3 with a manual transmission. The 991 GT3 RS will be an even more limited model so why would it make financial sense now? Especially considering the PDK 991 GT3 would outperform the GT3 RS unless it gets a significant power boost and loses considerable weight. Even then, the PDK version of the GT3 RS would just stomp the manual.
If Porsche were to offer a manual and the only way to get it in the 991 GT3 would be to opt for the GT3 RS that would definitely be a reason for purists to consider the model but it would also upset 991 GT3 owners. Not to mention if they can do it for the RS there really is no reason they can not do it for the standard GT3 at that point.
The official stance is PDK only for the GT3. Until anything of substance is cited otherwise from an official source this is nothing more than a rumor to generate traffic on par with the new M3/M4 supposedly not having a manual transmission option.