I don`t think that`s the reason. The old Sylvania "Fluorescent" decorative lamps are actually negative-glow lamps using neon and argon gas fills, and as far as I have been able to tell, green or blue phosphor coatings to generate the different colours. For example, the yellow ones use a predominantly neon (red-orange) gas fill with green phosphor. The orange one uses a neon fill with a plain white diffusing coating. The lamps are driven using simple wirewound resistors located in the bases, and at least using a Variac, they dim down to zero perfectly well.
I was not previously aware of the "no dimmers" rule, but can only imagine this refers to the simple two-wire triac dimmers intended for incandescent lighting. Those things usually do not work well with anything other than a resistive load, which the Sylvania lamps do not offer. That and the tiny power consumption, which is usually below the minimum load for those dimmers. If you had a long enough string of those lamps (to make the minimum load) and wanted to be able to dim them using a triac dimmer, I would imagine putting a single incandescent bulb in the string may offer enough of a resistive load to allow the dimmer to work at lower levels. A similar problem is encountered when trying to switch/dim LED light strings using some triac based controllers, and apparently the use of one "C7" 120 volt bulb in parallell with the string usually does the trick.
