Anyone saying Dekker is a comparable prospect to Van Horn either wasn't around when he was drafted, or has a fuzzy memory.
When a comparison is used it means the said player's game is reminiscent of the one he's being compared with. It does not mean he will be better, worse, or the same.
Van Horn was the far superior college player player, no question. However, that might not necessarily mean his game will translate better on the next level. Dekker's style of play, is reminiscent of Van Horn's, but Dekker has a better ability to defend his position (SF) on the next level. Van Horn was sort of caught in no man's land because he didn't move as well laterally as Dekker and this caused him to struggle defending quick forwards.
Let's not get pedantic about the definition of "comparable". In this context, it meant of similar/near/close talent level. I don't think they are close at the same stage in their careers, and people are way underselling the quality of Van Horn's career.
Dekker is maybe the 15th best prospect in this draft, with 3 years of college play that support the idea that he's not a lottery talent. To say he has a few big games and some defensive potential makes him a similar prospect to a guy who went #2 after Duncan and immediately stepped in as a 19-and-7 player is outlandish, IMO. He's not a defensive stopper by any means, just possibly decent at the NBA level. Van Horn would go top-5 in this draft based on his college career, even with the same defensive concerns.