I will agree that Lumiya's advantage was rather understated, though significant. For, earlier in Tempest, a no-name Jedi by the name of Tresina Lobi (no name in comparison to Luke) was able to engage Lumiya and Aleema (a dark Jedi) in combat simultaneously. Lobi more than held her own against these two, and would comfortably beat either of them in a straight up fight (think of Maul vs. Obi-Wan and Qui-Gon).
However, as pointed out, Luke's power varies from author to author. For example, Timothy Zahn was not pleased with how certain characters were becoming like "Supermen", including Luke (though it is ironic... he tries to put a no-name dark Jedi on par with the Emperor), and so for the novel Survivor's Quest (which is several, several years after DE), he arranges a situation that pits Luke and Mara up against a slightly modified droideka (a single one), and they have immense difficulty between the two of them to kill it.
Unless it changes, I consider DN/NJO Luke to be the strongest incarnation of Luke. And, though I think he'd defeat any other SW character in single combat, I think Glentract has a point - he's not skilled enough to breeze through everybody.