

I have no idea exactly what that means.
But Apple provides extensions for most functionalities, but, as you mentioned, they’re more limited because Apple used to require that extension developers register a $100 per year account in order to develop extensions.
They don’t do this anymore, but it was a big reason why Safari got held back, especially in the beginning of the browser wars.
That’s not holding back browser development, that’s just holding back browser usage.
That’s definitely not the same thing.