How is this different from an agency?

The deliverables can look similar. The starting point is not the same. Agency work usually begins with what to produce: how many pages, which channels, how much content. Advisory work begins with which layer the problem is in, and whether the work should be done at all.

In practice that shows up twice. First, time goes into inventorying the facts and assets that already exist, flagging the claims that contradict each other, and keeping the unevidenced ones off the page. That step usually reduces the work rather than adding to it. Second, engagements that will not accept fact-checking do not start, because a conclusion resting on figures nobody can verify is not worth building.

One practical difference: this is one person, not a firm. That limits how much runs at once, and it means the judgement and the delivery come from the same place.