Good ML people I know or have observed through interviews are good at a bunch of things. The ones that come to mind are: Linear Algebra, Statistics, Python, ET

Good ML people I know or have observed through interviews are good at a bunch of things. The ones that come to mind are: Linear Algebra, Statistics, Python, ETL operations with databases, Application Performance Analysis and testing, and the crucially overlooked and slightly more fuzzy what I call 'problem space analysis' (ie, looking at the problem space, whether it's a game or a process, and discovering which human learned activities can be effectively replaced or reimagined by training a model of some kind).