If you read the article you'll see that SSDs are actually, constantly purging old sectors to improve performance, and that it's only a short time before the old data is completely destroyed.
From the article:
After examining an SSD for traces of data after it had been quick formatted, the team expected the purging routines to kick in around 30-60 minutes later, a process that must happen on SSDs before new data can be written to those blocks. To their surprise, this happened in only three minutes, after which only 1,064 out of 316,666 evidence files were recoverable from the drive.
This means that if you write your new encrypted wallet to disk, one of two things will happen:
(1) The data is overwritten in place : GOOD
(2) The data is written to a different sector, but the old sector is purged within 3-60 minutes : GOOD
So you're really only vulnerable if someone steals your SSD within an hour of you encrypting your wallet. By then, the data should be completely gone.