In josekipedia, they state that this exchange is a mistake See http://www.josekipedia.com/#path:pdttpfqcpcqdqepbobrbpaqaqb for the full sequence they argue with. But, I think that their argument ...
With the three black stones placed, I did the 3-3 invasion. $$Wm1 $$ ---------- $$ ........| $$ ...X....| $$ ...X.1..| $$ ....X...| $$ ........| I tried tried to force a KO and did get 2 moves ...
I have read somewhere, but unfortunately forgot where that the following enclosure is safe against the invasion at a. $$ White invades at 'a' and supposedly dies. $$ --------- $$ -........ $$ ...
The two-space extension is a very common pattern and forms a base. I've found lots of material here, and here, and elsewhere on how it cannot be cut and how to defend against various invasions. ...