At the beginning of their response, they basically say the can not do it. However, at the end they say that if I insist on it, they can.
No, what they're saying is if you want to do this, you'll have to do it elsewhere, not on their nameservers, as they offer only a basic stripped-down service that can't do it.
So, in Moniker set the custom nameservers to your web hosts nameservers (avoid Moniker's service completely). At the webhost, set up www etc to point to the IP of the webserver (probably already done). Then, ALSO at the webhost's nameservers, add MX records to point to the external mail service.
If you don't have a DNS control panel at the web host they should be able to add the MX records for you. If they can't, use an external DNS service (paid or free), and point the custom nameservers at Moniker to that instead.
To clarify, there are several things here. One is domain registration (Moniker), which points to nameservers. Those nameservers can be anywhere, from any company, they contain records that point to your web server IP and the probably different MX records for your mail server. Then you have a web host, and a mail host.
Any of these can be combined, but it's quite possible to have each of the parts at a different company (I do). One thing you can't do is combine domain registration and (flexible) nameservers at Moniker, as they don't offer it.