Is an AWS SQL Server RDS "log capture" the same as a transaction log backup?
Almost certainly it does trim the transaction log. (Otherwise the Amazon cloud will fill up with transaction logs.)   

Some details from the Amazon site include this topic and two sub-topics that you can investigate: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_SQLServer.html


  When you create a DB instance, you are assigned to the db_owner role for all databases on that instance, and you will have all database-level permissions except for those that are used for backups (Amazon RDS manages backups for you). 


For a discussion of the backup model and how you influence it, see:
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.SQLServer.CommonDBATasks.html

And, of course, if you change the model from FULL to SIMPLE, the retention period, and so forth that will affect your restore points.  

Amazon definitely support point-in-time restores, which means that the transaction logs are being used.  But since they do this for all database server types that they support, I do not know the mechanics of what they use to do this.

On the restore process described here: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIT.html


  When you restore a DB instance that is using the SQL Server DB engine to a point in time, each database within that instance will be restored to a point in time within 1 second of each other database within the instance. Transactions that span multiple databases within the instance may be restored inconsistently. 


So I would say that, whatever the mechanism of the log backup, it will be available within the constraints discussed in their documentation.