You are right. I was mistaken.
It is
not particularized in the TR, which was what had me confused, but that often is the case (that a rule should not be particularized in the TR, not that I should be confused). Changes to F1 rules often are handed down as letters of clarification to the teams or instructions to the stewards and don't formally alter the SR or the TR (such as the M. Shumacher-inspired one blocking manoeuver rule).
I had to dig to find that the rule was changed in 1995, after which the driver
is included in the spec weight. It never occurred to me to question why they bothered to weigh drivers after the race.
But that actually makes the driver's weight
more critical since it represents a larger percentage of the total weight. Daniel Ricciardo is several kg lighter than M. Webber, but his bum proved too wide to fit into Webber's cockpit. Which must be making any number of weight loss/fat-burning supplements looks attractive to him.