I am unable to confirm this.
I downloaded six different sets of files and downloaded all via IDM in one VM and via FDM in another VM.
For *all six*, FDM made the downloaded files "available" *LONG* before IDM.
I haven't used IDM enough to know if this is "always" the case, but *after* 'downloading' a file, it went into an "appending all pieces into one file" routine.
FDM did not do this, the file was *available* immediately after the download 'completed', I didn't have to wait, and wait, and wait for the pieces to be "appended together".
I don't do a "ton" of downloading, but when I do, the files are LARGE and sourced from high-seed-count torrents.
So I just let my "portable" torrent program do the downloading. I've not technically compared/contrasted my torrent program to IDM/FDM.
But from what I just witnessed with six different sets of files, I would lean toward FDM over IDM.
I didn't compare any other download managers, just compared these two. Truth be told, for MY download needs, I don't like either one, I prefer my "portable" torrent client instead.