By most definitions, Treasure Island is not historical fiction. It is only historical fiction in that it was set in the past and to some degree in an important time in history.
The most common definition of historical fiction is that it must contain some real life characters and situations. These are usually from some very important time or event in history. Stevenson's book Kidnapped is more like this than Treasure Island. Treasure Island has no figures from actual history. It is not set on a real island. Instead, it is simply set in the general time period when piracy was still common.
This book, then, is only historical fiction because it was set in a time before it was written and with characters who could have been (but weren't) real.