Still, if he could telepathically control literally every sentient being in the galaxy, he would've been unstoppable and he would've succeeded on virtually all of his endeavors. As we all know, in the end, he wasn't and he didn't.
This was in reference to the DE endnotes where his final plan was to turn the entire Galaxy akin to the DS nexus on Byss, which everyone in the galaxy basically feeds him their force energies and what not and planned to rule other Galaxies.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Not really. He based the Dark Empire on the principle that it would "follow the Dark Side's will." He intended to take over the galaxy and, since the galaxy's population would be under rule of the Dark Empire, every being would have to follow the will of the Dark Side, and the quote could be interpreted to mean that his goals were achievable (which they clearly were, if the Skywalkers hadn't been a factor). Admittedly stretching here, but it seems extremely unlikely he would not use this power if he was capable of it. It would've meant he didn't even need to use the galaxy gun on Da Soocha, he could've casually dominated the planes population. He also could've easily dominated Luke and Leia since they had nowhere near the training or feats necessary to imply they could handle TP strong enough to slave the entire galaxy.
Hell, it's possible this quote is simply saying he's capable of using a ritual to dominate the galaxy.