For example, the Rotax 912ULS engine has a 2.4:1 reduction. Does this mean that the Time Between Overhaul (TBO) will be reached 2.4x faster than in a direct drive engine or is tach time reduced by 2.4:1 as well?
The relationship between tach time and RPM varies based on the engine. As mentioned in the linked answer, it is difficult to find this relationship based on engine specifications, but if you find a tachometer certified for the engine it will have the correct ratio built in.
If the 912ULS needed TBO based on tach times, the tachometer would likely count tach time at 1:1 somewhere below 5500 RPM, since that is the maximum continuous speed for this engine. However, Rotax actually uses Hobbs time to determine when an engine needs a TBO, and specifically notes that a "mechanical hour meter" may give different times. Therefore, the TBO of 2000 hours is 2000 hours of real-world time.
From the 912 Series Maintenance Manual: (p. 3)
All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate to the number of operating hours of the engine.
The operating hours are defined as follows in order to prevent misunderstandings and to ensure safety:
All time during which the engine is running is counted towards the total number of operating hours.
The time is counted irrespective of the load factor of the engine, such as idling or take-off power.
NOTES: A mechanical hour meter is directly coupled to the engine speed, the readings may deviate considerably from those given by electronic remitters (e.g. TCU, FlyDat). Maintenance and overhaul intervals are always dictated by the readings of the electronic hour meter.