That is actually not quite correct. I believe it would be better worded as "Your web server is configured to block the bots (or any automated attempts to fetch content), so you will not be able to use HTTP verification unless you turn that protection off temporarily. You can use DNS verification however, as long as you control your domain records".
This case is not specific to ZeroSSL and this has been previously responded here - Invalid Response From... even when file is ok - #3 by leader