Sep 17, 2018 · What you’re seeing is completely normal. WS US will not show you the actual cumulative updates for exchange in WS US only the security updates for the cumulative update. Also and you may already know this but each cumulative update for exchange is a full version of exchange. What jrp78 is saying is correct. WSUS will not upgrade exchange CU's.
To get the latest version of Exchange 2016, download and install Cumulative Update 18 for Exchange Server 2016.Because each CU is a full installation of Exchange that includes updates and changes from all previous CUs, you don't need to install any previous CUs or Exchange 2016 RTM first.