Their report says:
Some members of the astronomical community have expressed great concern over any change to the current system. These concerns are based on the use of existing software that takes advantage of the current definition and uses UTC as a substitute for UT1. Their requirements for precision are such that the current 0.9-second tolerance is adequate, and their software has been designed accordingly. Should the definition of UTC be modified in any way that would permit this tolerance to be exceeded, they anticipate it would create a substantial cost to make non-trivial changes in existing software. Some members of the astrodynamic community have voiced similar concerns regarding legacy software used in the determination of orbital parameters of artificial satellites that again utilizes UTC as a substitute for UT1.[1]
So I'd suggest the FB summary of their objections is pretty much correct.
Reading the report completely, it seems the IAU wants to create a new time standard (not UTC) that drops the leap second and keep UTC as it is:
It was suggested that a means of transitioning to a uniform time scale could be accomplished by the creation of another time scale that might be called Temps International (TI) to clearly distinguish it from Universal Time.
[1] https://www.atnf.csiro.au/iau-comm31/pdf/ContDoc-010_IAU_060...