It would depend on the version of Wordpress, in general the idea is that you need to patch your software and harden it to make it secure. But it's not that Wordpress is any less safe than Sharepoint.
Wordpress is in general well maintained and CVE's are patched or fixed quite rapidly. So if you keep up with these then it's safe to use Wordpress. (do mind that you need to keep track of your plugins as well)
If you don't do this in general, then your Sharepoint will probably be as insecure as your Wordpress (or any other software that's on your Intranet for that matter).
But saying that software X is insecure without providing any facts to support the statement is a bit stupid.
To compare intranet vs internet, the likelyhood is lower, but the impact is the same as on the internet.