In an earlier
PR describing the recorder offloading procedure, it was noted that France was concerned that the Brazilian government may impound or restrict the movement of the recorders if they were taken to Brazilian territory.
The fueling of the French patrol boat in Brazil prior to it beginning the trek to the recovery site may be a precaution to give it enough fuel to make it to the recovery site and then direct Guiana.
I hope they are planning on flying the boxes on two different flights to be conservative.