yes, it's an isolate. it's unclear if we'll ever be able to connect it to another language simply due to time. the same is true of Turkic, Mongolic, Korean, and Japanese. This is also true of Indo-European; there just a huge time-lapse and its closest relatives might have been lost.
the other thing about the Altaic theory is that it isn't really one. No two people who argued for it back in the day even agreed on which families belonged to it!