Tonne is a mass measurement... How can 1 tonne store 73 tonnes of CO2?
The source of that fact is here [0], but that is counting the forest that makes the cork. However they aren't growing a new forest for new cork, so that 73 tonnes of stored carbon is static. Each cork harvest lowers that 73:1 ratio.
https://www.amorimcork.com/en/media-center/news/research-hig...
> Planting forests is a commonly used approach to offset carbon emissions by polluting industries, but when the trees are harvested they are usually cut down and much of their stored carbon is released back into the atmosphere.
> But cork oaks are one of the few commercial forests not felled for harvesting. This means the cork tree's carbon storage capacity keeps increasing during the 200 or more years the trees can live.
> Most of the carbon remains locked in the tree as it continues to grow. Although cork products contain some of the absorbed carbon, they can have a long life after being cut from the tree. Cork can be recycled and is slow to break down even when discarded.
There's about 2.5g of oxygen per 1g of carbon in carbon dioxide, and carbon takes the lion's share of hydrocarbons' mass, so I wouldn't be surprised if just the cork itself would burn to twice its weight in CO2. Then you consider the growth of the tree itself over time. Oak is about 3-4x the density of cork, so I wouldn't be surprised if you got a few grams of wood per gram of cork.
That 70x multiplier is enormous and probably overinflated, but these are just two factors I could think of in a couple of minutes, so I can easily see how a tree, over its lifetime, might sink 10-20 tonnes of carbon per 1 tonne of cork produced.
(Note I am not attempting to state whether that claim is true or not, merely offering one potential explanation of how the trees might sequester 73 tonnes CO2 for every tonne of cork harvested)