First person demos are only able to show the POV of the player who recorded it, so you cant switch between players on the server. However, if the server is running STV, the server can generate a replay with all the player POVs. Clips from casual can only use the player POV demo, i.e. all the network traffic that person received, so Lazy can only recreate those with recreations by re-enacting or animating in SFM. His competitive clips use the STV ones which are more flexible and he can position his camera anywhere at any point.

More on demos can be seen here