I am not asserting anything - I haven't stated that AA has implemented married segment logic - but others have reported it

All I have done is describe how married segment logic works

How many seats AA makes available is irrelevant to this; as far as making a change goes ,
If changing flight A of A-B-C ( with no change to transit/stopover) , there just needs to be availability on the new flight A-B for a connection to B-C

Availability for B-C doesnt need to be searched for , since it is already held
What will not be of any use is A-B being available for standalone - only where A-B is available for A-B-C