in CmdStanPy, specifying threads_per_chain=88 sets environment var STAN_NUM_THREADS=88, and evidently, this does the right thing w/r/t reduce_sum, which is why you see 88 cores working.
so yes, probably safe to ignore these messages - maybe it’s a ubuntu thing? cf https://stackoverflow.com/questions/43641247/numexpr-detecting-number-of-threads-less-than-number-of-cores - because as far as I can tell, neither CmdStanPy nor the c++ code or makefiles set or use NUMEXPR_MAX_THREADS