The grammar here is a bit more complex, and it looks like that's confused you. Let's take a closer look at this structure.
There are a few things going on here. If we diagram it a bit differently, it might become more clear what's going on.
- The simplest core of the main statement is that top line: ５年間【ごねんかん】を[振り返る]【ふりかえる】 ("to look back on [those] five years"). That's where that を fits in -- it marks the object of the verb [振り返る]【ふりかえる】.
- The second line is a modifying phrase, used to tell us more about what kind of ５年間【ごねんかん】 we're talking about: そんな濃密【のうみつ】な ("those rich and full / jam-packed ...").
- The third line is the simplest core of the embedded sentence: コラムを交【まじ】えつつ ("while mixing in [newspaper?] column [content / articles / pieces]"). This has the ～つつ ending, telling us that this action is happening alongside, or in a subordinate way to, the action of the main statement.
- The fourth line is another modifying phrase, telling us more about what kind of コラム we're talking about: 記事担当【きじたんとう】の ("belonging to the person in charge of articles" → "the editor's").
Putting it all back together, we might translate this as something like the following. This assumes that the context is a newspaper, magazine, or similar medium, that would have a regular feature section, in which this text appears.
We look back on those jam-packed five years, with a mix of columns from our feature editor.