The problem is that Lumiya was but an Emperor's Hand; like Mara Jade, she was only taught and shown precisely what Palpatine wanted her to see. I refer you to Mara Jade's testimony in the Hand of Thrawn duology where she claims that Grand Admiral Thrawn possessed superior political acumen than Palpatine himself.
The Emperor had a notorious history of limiting the knowledge of his subordinates; he is on record to have limited Tyranus's knowledge of the Sith; he cut dark side adept and Inquisitor Jerec off from the Dark Side Compendium; Vader's own musings in the Rise of Darth Vader conclude that Palpatine doled out "precious bits of Sith knowledge" that kept Vader hungry for more but not enough "to make him supremely powerful."
Logic dictates that Lumiya was relatively ignorant of the true scope of Palpatine's power (even though she'd later go on to tell Jacen that the Emperor's power was "limitless."😉. Furthermore, Jacen's five-year-sojourn across the backwaters and fringes of the galaxy would certainly yield a startling amount of knowledge, especially when compared to Luke Skywalker who had a laughable knowledge base to begin with and was active in the New Republic military, which largely prevented him from going on such treks.
But to compare him to Palpatine is also a joke; the Emperor spent twenty years as the galaxy's sole dictator of the galaxy's sole superpower. He "gathered the greatest works" of Force knowledge from "a million worlds" and heaps of Sith artifacts and arcana. He was a superb alchemist, having Naga Sadow's knowledge as well as having mastered the Krath (according to the Dark Empire Sourcebook), not to mention the reservoir that is the Telos and Tedryn Holocron.
As far as that goes, let me say explicitly that Palpatine has demonstrated the greatest command and knowledge of the Force out of anyone in the mythos ever. No one compares.
Though I will say that it's also a bit of a chuckle to think that Darth Revan's relatively minor stint with the dark side is enough to yield truly uber Force knowledge. His fanboyism knows no bounds.