Yes it is very much true. Any modern oversight of slavery should be able to tell you this. In my case I read the 3 volume work "Slaveri "(2008) by Swedish historian Dick Harrison covering slavery from ancient times to its modern expression all over the world. Yes it is a mastodon work.

Caravan routes through the Sahara brought gold and slaves north since ancient times (and in the Eastern part the Nile was the primary artery bringing up slaves) and it was in part to get access to this trade directly that Portuguese navigators searched for a way around the African west coast.

Slavery in Africa tended to be more domestic than the slavery later leading to the transatlantic trade, mostly coming from prisoners of war and debt bondage. It was basically endemic, existing everywhere forming the natural state of being for most people. That is to say it was accepted as how things were (like basically every other place in the world) though naturally if it happened to you personally you would try and avoid it. Trade routes ferrying slaves around Africa long predated the ocean going slavetrade.

When European traders arrived their primary motivation wasn't even slaves, they wanted gold and after that other exotic goods. But they also found that there was profit in trading slaves, at first locally to get gold and exotic goods, getting slaves from cheap areas and taking them to places of demand. As plantation economies developed, first on the islands off Africa and then on the other side of the Atlantic slavetrading rose in importance as primary economic motor for the trade.With an almost insatiable demand for colonial goods in Europe, high death-rates of slaves in the Americas the transatlantic slavetrade grew quickly in size. The existing slaveroutes of Africa started to feed into a basically endless market demand for slaves and local kingdoms found a great source of profit and power for the elites (monopolizing or otherwise protecting and furthering the slavetrade). Partly from crass economic interests, but also from a political angle where either you participated in the trade or became its victim (with some exceptions). With slave trade came better weapons with which to increase your power and profit over your neighbours e.g. in the region the Portugese were heavily involved with, Congo, Angola (modern) where the rapacious slavery expanded inland as nation after nation adapted to the "system".

In conjunction with the transatlantic slavetrade which concerned mostly Western and southern parts of Africa, the ones connected to the ocean by river basins ending in the Atlantic there existed an Eastern African slavetrade up to Egypt via the Nile and along the Eastern Africa coast. This existed before and after the transatlantic slavetrade flourished. Only in the latests stages did these system connect into each other.