51. Amazon Lightsail
51.3. You may not use Amazon Lightsail in a manner intended to avoid incurring data fees from other Services (e.g., proxying network traffic from Services to the public internet or other destinations or excessive data processing through load balancing or content delivery network (CDN) Services as described in the Documentation), and if you do, we may throttle or suspend your data services or suspend your account.
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.
