The smarmy 'spokesman' of the Think Tank (from the episode of the same name) claimed he and his helped find a cure for the phage. Now Voyager's a long way from Vidiian space by now and I wouldn't put lying about that past him for a second- was any material confirming or dispelling this claim published/released?
Possibly...but probably not
The entry on the Vidiians at the official Star Trek web site, StarTrek.com, makes no mention of the phage being cured at any point :
A space-faring Delta Quadrant species known as educators, explorers, and above all artists — until the phage hit them two millennia ago. Thousands die each day, and organ harvesting is the only way to survive; corpses are preferred, but in an emergency they may be taken from a living body. As a consequence, their medical technology is much higher advanced than the United Federation of Planets norm, including immunogenicity. They are not familiar with holographic imaging, and their power systems may use a dilithium matrix. A honatta helps a patron find the appropriate organs. Despite the disease and lack of conscience in this area, the culture somehow goes on: Motura is a sculptor. The disease leaves them weakened, requiring slave labor to do such tasks as mining.
There is no mention of the cure, as rumour, fact, or otherwise.
Similarly, the official entry on the phage itself does not mention a cure. It seems that the writers intended for the truth behind Kurros' claim to be ambiguous at best.
Now consider the following points:
- There was no evidence of activity by the Think Tank when Voyager was closer to Vidiian space. (None of the cultures encountered by Voyager had reported contact with a group of "problem solvers".)
- The Think Tank do not seem particularly mobile.
- Three years worth of travel at high warp had passed between Voyager's encounters with the Vidiians and meeting the Think Tank. (In that time they had also been propelled psychically across a vast distance by Kes, and then catapulted again by an advanced alien technology — thanks @Tritium21.)
Taking these points together, it seems unlikely that the Think Tank would have crossed paths with the Vidiians. Given the group's advanced technology, they may have simply scanned Voyager's database and picked something that would impress Janeway — a solution to a problem that seemed unsolvable.
As @Politank-Z points out in his answer, there are novels in which the phage is cured at a time that is compatible with Kurros' claims, but of course that is Beta material (i.e. not canon).
By the beginning of the 2380s decade, the Vidiians had rid themselves of the phage. (TNG novel: Q & A)
The Think Tank's role in this is ambiguous, at best. The encounter with The Think Tank was in 2375; The Vidiians could have found a cure not involving The Think Tank by then, but after 2000 years of The Phage, it would be a coincidence. Then again, coincidence in Star Trek is awfully common. Perhaps Klingon physiology provided the key.
Yes, they did (probably)
Their aim in the Voyager encounter was to recruit (note, not abduct) Seven of Nine into their odd little collective. They see her as a potentially willing new member and it would hardly be sensible to make unfounded boasts to get her to join.
As a member of the Think Tank she would learn of their deception in very short order, pretty much as soon as she has access to their files:
SEVEN: Why me?
KURROS: I admire your humility. The collective knowledge of the Borg contained within a single mind. When we factor in your innate intelligence and the enhancements afforded you by your bionetic implants, we concluded you are an ideal candidate. The first we have recruited in seventeen years.
SEVEN: When did you arrive at this assessment?
KURROS: During your interview.