Is the theory of hot spots compatible with lengthy volcanism?
Tectonic plates are not single blocks of geology. The Africa Plate, like other tectonic plates, consist of cratons  which are "stable blocks of old crust with deep roots in the subcontinental lithospheric mantle, and less stable terranes, which came together to form plates".

The Africa Plates contains the "Kalahari craton, Congo craton, Tanzania craton and West African craton".

The island and volcano of St Helena, though on the edge of the Africa Plate, is more associated with the Mid Atlantic Ridge than it is with the Africa Plate.

The cratons and other land masses that form the Africa Plate may be mobile, but St Helena and the Mid Atlantic Ridge are not.