I'm a bit doubtful about this because I know that even German mills started to produce lumber with US dimensions, causing shortages for metric lumber locally.
But, if there was an expectation that these price levels were going to be sustained, the mills would have started engaging in capital programs to increase their capacity - because, and this is critical - there was no supply of raw-timber issue, indeed - the price of raw timber didn't increase.
That those capital programs did not take place, is the strongest representation I can think of of Mill owners true assessment of the long term prices.
If they thought this was permanent, they would have expanded to supply the USA, rather than doing so at the expense of domestic production.