MATLAB uses Name/Value pairs in which the first element of the pair tells the option name and the second element of the pair indicates the state to use.
MATLAB does not use option keywords (that force behaviour if they appear by themselves). Keyword options are harder to parse. If you use name/value pairs consistently then each routine can parse for the options it is interested in and can ignore the other options; but if you permit keyword options that take effect just by appearing alone, then every parsing routine that deals with those options would have to know the complete list of keyword options so it would know whether to advance by one or two positions to keep looking for more options.
So, 'PreserveVariableNames', true means that names are to be preserved, and 'PreserveVariableNames', false would mean that names are not to be preserved (which is currently the default.)