It's for interaction with existing rules.
So, our ability here reads thusly:
Brain Drain (Su): You can take a standard action to violently probe the mind of a single intelligent enemy within 100 feet. The target receives a Will save to negate the effect and immediately knows the source of this harmful mental prying. Those who fail this save are wracked with pain, taking 1d4 points of damage per oracle level you possess. After successfully attacking with this ability, you may use a full-round action to sort through the jumble of stolen thoughts and memories and then attempt a single Knowledge check using the victim’s skill bonus. The randomly stolen thoughts remain in your mind for a number of rounds equal to your Charisma modifier. Treat the knowledge gained as if you had used detect thoughts. This is a mind-affecting effect. You can use this ability once per day at 1st level, plus one additional time per day at 5th level and for every 5 levels beyond 5th.
I've bolded a sentence in there because it's kinda important. What Brain Drain does is make an attack against a creature, then give you another ability that you have to use up within a number of rounds equal to your Charisma modifier - chiefly the ability to make a Knowledge check using your victim's skill ranks. If gaining the knowledge was supposed to be similar to detect thoughts in the ways your question describes, there would not be a time limit.
The contentious sentence, "treat the knowledge gained as if you had used detect thoughts," is for the purposes of spell and ability interaction, chiefly spells and abilities that render a subject immune to mind-affecting spells, that block detect thoughts, or that trigger upon psychic contact (of which detect thoughts is a frequent example). Note the lack of tagging on this power, which should be listed as mind-affecting? Paizo has used the somewhat uncommon, but observable, wording of treating one ability like another, earlier-published ability in order to resolve interactions within the rules.