It sounds like this developer doesn't know what he's talking about. The security venerability is there whether or not you Jailbreak. Jailbraking just takes advantage of it.
Jailbreaking can cause a security hole if you don't change the root password. It's very easy to change so there is no reason not to, so this shouldn't be an issue.
Jailbreaking can be a problem if you install software from fishy sites, but this is true for all computer devices. So again, this shouldn't be an issue.
Jailbreaking can make your device more secure, because there have been instances where the only way to close the security hole used by the Jailbreak is to actually Jailbreak your device.