The clause prohibits you from using Lightsail to cheat other services. So, per their example, you couldn’t set up a Lightsail instance as a reverse proxy between the internet and an ELB, to take advantage of Lightsail’s higher transfer quotas and “in-region” traffic from Lightsail to ELBs.
Hosting a site on Lightsail and hosting other things on other AWS services is fine.