It's nick-pickin, but technically the guanches were only the habitants of Tenerife. For a generic term it would fit better to call them "Ancient Canarians". It's also wrong to consider them a stone age culture. They used stone to make tools since the Canary Islands lack metal deposits, but they had a well developed culture with writting and complex societal structures, akin to the prothistorical berber cultures of the Maghreb, from where they arrived in the Antiquity.
They certainly had some contact with the outside world, sadly most often in the form of raids from the Maghreb to get slaves. They arrived to the islands in historical times, for unknown reasons and probably in several waves, and cut ties with the outside world also for unknown reasons (it would do an excellent speculative fiction, renegades that try to create a new society in some unhabited islands but thigs doesn't go as expected, but this is me dreaming...) They also knew basic navigation, as recorded by the castilian conquerors, they crafted small rafts they used to plunder neighbouring villages and islands. After all from any of the Canary Islands you can see the closer islands in clear days, it would be strange that human curiosity doesn't get bailed by this. It's difficult to get them "uncontacted" forever, but certainly if the events that led to the iberian maritime explorations are butterflied or changed, and the spice market is more easily avilable, for example, they could stay with their traditional life style during more time. But it's not only the castilians. The are very close to the african coast, so sooner or later....