How can a completed match become a walkover?

Here's what Wikipedia article on the term walkover suggests:

A walkover is the awarding of a victory to a contestant because there are no other contestants or the other contestants have been disqualified or have forfeited.

So,

How can an already played match, lost by one of teams, later be declared walkover?

Easy, if it's later identified that a team fielded an ineligible player, or due to crowd violence, etc.

Side question: About exactly which match Polish commentator was talking about?

As @Martin pointed out in the comments, most probably the commentator was referring to the incident of Fernando Varela playing for Cape Verde against Tunisia during his suspension.