This non-standard event property has been deprecated. One should either try the standard property first and keep the fallback, or even ignore the deprecated variant entirely if the fallback without can be considered graceful degradation as result of progressive enhancement.
Depends on how visible the impact is and which version of which browsers support it. It seems to WebKit-only in the current implementation, which means the feature may be broken right now in Firefox (which supports Event.movementX as of Firefox 41). – Or perhaps this check is for a Chrome-specific bug of that feature rather than the feature itself?