That's true, consequences of actions are only ever felt within a very limited timeframe.

Brexit took close to 5 years from vote to implementation, during which there was lots of noise from all sides. The Softbank deal came after the Brexit vote, once the Sterling had dropped sufficiently (~30%) to make it additionally enticing.

Among articles from the time, https://www.forbes.com/sites/parmyolson/2016/07/18/arm-softb..., complete with government spin.

It's hard not to make something about Brexit, when the government explicitly chose to make the thing about Brexit. The Softbank deal was disastrous for UK tech, and if the government really was interested in national security grounds (which is what they're claiming this time around), it would have intervened in that deal. NVIDIA isn't likely much more of a risk from a national security perspective than SoftBank. But it had to sell Brexit, and a massive foreign investment in a UK company was an opportunity for spin too enticing to give up.

The full legal consequences of brexit have still not fully come into force yet (there are various grace periods still to elapse: the most recent was the start of this month and there is still another extended to 1st October). And the secondary effects will likely take years to become apparent.