But they have diverged so long ago, they are for all practical purposes different rendering engines. It’s not the same deal as, say, Chrome and Edge.

Right, Blink and WebKit are no longer reliably similar, which is why when devs don’t test on WebKit (to say nothing of Gecko), rendering stuff can reliably break.

They share a ton of history but the split happened eight years ago and A LOT of code has changed. In fairness, WebKit (and thus, Safari) has later adopted some of the same changes, but they really aren’t synonymous anymore.

Blink and Webkit have become separate enough that you can't call them the same in good faith: layout engine is completely different, style engine is different and many other parts have been rewritten in both Blink and Webkit independently of each other.