The whole (rather breathless) point of the article is that SSDs erases the blocks quite rapidly. It isn't that the old blocks are still laying around but the researchers did not know how to access them, it is that (a) they are erased rapidly under normal conditions and (b) they are erased even when they researchers used a "write blocker" mechanism[1]. IOW, they (the article's author, anyway) were apparently surprised to find that the SDD garbage collected autonomously and regardless of the "write blocker" mechanism).

The (a) case is likely OSes using the "trim" command and the (b) cases are inherent in how SSD firmware works (by necessity). WRT the (b) case, SSDs have to garbage collect. Their write speed, and thus user satisfaction, is dependent on it keeping a large number of erased blocks ready to be written to.

[1] http://www.forensicswiki.org/wiki/Write_Blockers - apparently simply a "man in the middle" hardware that filters out write commands, lets through read commands.

Obviously, there is no reasonable way for a SDD to even know that a "write blocker" is attached, so it is not surprising (to me) that the SDD garbage collects with it attached.