Less than operator not working on where clause in mysqldump
Put the options first.  That is, all parameters that start with - or -- before the non-option parameters (database  account) and redirection (&gt; ... or &lt; ...)

If that does not suffice, then remember that the command line processor is handling the first level of quotes, so change to

"--where=insertDateTime &lt; '2019-08-13 18:00:00'"


(but I am not confident that this will help)

The quotes are to deal with spaces that are not separators -- as in the middle of the path or the 'where' expression.