Ceramics are incredibly strong and hard but are not very tough (i.e. they initiate cracks easily), and are brittle with graceless failure, as the cracks propagate easily from the crack tip right through the ceramic.
So a ceramic is only as tough as its weakest point that experiences any sudden force; it's all about preventing a crack beginning in the first place, because once it begins, it's going to rip through the ceramic item.
Composite ceramics are much stronger because they have two phases of ceramic, generally one which is a matrix (or surround), with another one as a fibre or filler particles.
If the fibres/fillers adhere to the matrix strongly but not too strongly, then cracks that strike them don't propagate further. The tip "bounces" off the filler and the item is saved.
If adhesion is weak, then the fibres pull out and act more like holes than reinforcements. If adhesion is too strong, then the fibres cease to act like fibres and cracks propagate right through them just like through an unreinforced ceramic.
All this is to say that when you are talking about reinforced ceramics, the magic lies usually not in the bulk chemical composition (although minor additives can be really important), but in the morphology, i.e. 561
the distribution and adhesion of the filler, along with the all-important defect reduction during production.
Re: aluminium oxide (Al2O3), the most common form is alpha-alumina, which is also the strongest and form. What would have made that ceramic exceptional would have been the successful insertion of defect-free SiC fibres into defect-free Al2O3 with just the right adhesion strength and in some way that resulted in useful shaped items (you don't get to machine them afterwards).
In the 1980s, it was pretty hard to make long ceramic fibres, all they had was short fibres. I'm guessing the fibres in question would have been unusually long and defect free.
FWIW, porcelain (real porcelain, not white stonewear) is similar to an engineering ceramic; it's got long mullite (engineering ceramic) needles in glass, which is why it's so much tougher and able to be made finer than average stonewear. You can compare them using a microscope. 561