That is the system sort, not samtools sort, because a variable in the command was empty
-@ is a samtools flag for thread count; the sort that ships with macOS and BSD has no such option. Seeing this means the shell ran its own sort. The usual cause is copying a command out of a pipeline log, where tools are referenced through environment variables that only exist while the pipeline runs: $SAMTOOLS sort -@ 4 with an empty $SAMTOOLS collapses to sort -@ 4. The pipeline is not necessarily broken. Your shell is just missing the variable.
Do not run commands copied straight out of a pipeline log. To reproduce a step by hand, put the real program name back: samtools sort -@ 4 .... The same goes for every $TOOL reference in that log.