Well [thumbs through EU encyclopedia] ...
I think this line from Tarkin was chiefly designed to rile poor Krennic but I can explain it away if you would like [waves force hand at Khev].
In the PT and the EU (including Cataylst the RO prequel novel) it is pretty clear that Tarkin was not that involved in the initial construction of the Death Star. Dooku took the plans from Geonosians (the plans were unfinished and did not include details of super laser which Galen designed later) and gave them to Sidious. Sidious in his guise as benevolent Chancellor Palpatine then disclosed the plans to his top military advisers (excluding the jedi but including Tarkin) in a secret briefing where he said they had been intercepted from the Separatists.
Someone in the meeting (which may have been Tarkin thanks to that line in Rogue One) had the bright idea that the Republic should build their own Death Star to combat the Separatist's Death Star and this started an imaginary arms race (in reality the Separatists were never building one). Construction was slow and costly (I mean it took from the end of ROTS to RO to complete) and many believed it could not be done. It was also mired in the issue of the super laser which proved impossible to solve until Krennic (trying to advance his own career) convinced/tricked his old buddy Galen to work on it using Kyber Crystal research. At this point Krennic had weaseled his way into taking control of the construction of the Death Star.
What we see at the end of ROTS is Tarkin, the Emperor and Vader visiting Geonosis where the first stage of construction is taking place under Krennic's control (phase 2 of the construction and installation of the superlaser took place over Scariff).
Tarkin always held the belief that the best way to govern the galaxy was through fear (what us EU nerds call the Tarkin Doctrine) and upon seeing the Death Star at this phase in construction began to believe that this might actually be the thing to do it ... it's not a stretch to think that his line in Rogue One could also have been referring to some witty comment he made to the Emperor at that time like "Yo Sheev, this Death Star is exactly the kind of thing that we can use to govern the galaxy through fear". From that point on he kept a close watch on its progress and Krennic realizing the danger was constantly trying to out maneuver Tarkin and keep him away from his baby.
In RO Tarkin, upon witnessing the power of Death Star, decided to step in and take over / take credit for it. Krennic only ever reported to Mas Amedda during the construction process and the Emperor only visited the Death Star on that one occasion shown in ROTS. Tarkin had been with the Emperor on the occasions that he had either been in a meeting discussing the Death Star or visiting it ... so not a stretch to think that he thought he could easily swoop in and take credit it for it especially as Krennic was way down the pecking order and barely had the courage to whine to Vader let alone the Emperor.
No gaffs, no continuity errors all safely explained away by the EU.
So you see Khev the ability to ignore the PT is nothing compared to the power of the EU.
