The Amazon Basin is certainly a unique situation, due to the high rate of conversion of organic matter into living forest biomass.. there's literally no 'extra' organic matter left around in the soil, it gets sucked right up. In order to do agriculture, the organic matter tied up in the canopy had to go back into the soil.
With 'biochar' actually having a slower rate of breakdown cw other forms of organic material, it's a good fit for that situation - ideal really - to keep some organic matter in the soil as long as needed.
Sure, where I live we also have highly acidic clay soil. But with cold, wet, short growing seasons, a vegetable garden needs the stuff that will cycle as fast as possible in the circumstances. This alone would explain the detrimental effects of burnt wood in my garden.
Those of you in warmer climates might well benefit by tying up that organic matter in the soil, and will not mean you have to wait two years for vegetables.
In my environment, ashes also don't fix the soil pH for very long. So much rain just washes it out, and a year later the soil amended with ashes is more acidic than ever. Not a practical alternative to lime (no calcium in the soil here either) but a good source of potassium in the compost, afaik.