I find that hard to believe. Tamiya kits come with diffs at both ends, unless you get an advanced kit. Then sometimes, they come with a spool. In the case of the TA07, the pro comes with two diffs, the R comes with a spool and a diff.
Spools also require more expensive outdrives and drive shafts.
Now, I am also in the group that believe diffs at both ends is "faster". I believe the spool obsession is for "simplicity" and "stability" as opposed to outright performance.