Why AD connected RD Gateway in DMZ is not risky??

Hi

We are deploying 2012R2 remote desktop service. Our plan is that Gateway and Web Access will be in DMZ zone and others in LAN.

Because of that we should open firewall ports from DMZ to LAN which sounds creepy. Article below claims that opening ports is not too risky

https://social.technet.microsoft.com/Forums/windowsserver/en-US/0b5d95ec-94ea-40d8-9510-d01b018b688d/best-practice-for-rdgw-placement-in-rds-2012-r2-deployment?forum=winserverTS

Can somebody argue why it is not risky?

~ Jukka ~