Hi Carlos,
No Carlos, it was just an example of the evolution about how R3 got communication in the past and how ECC can get communication with PI now a days... when i say "Natural SOA" is just to express that this Architecture offer you more than just to think about RFCs (that even actually PI still supporting in this days), or others (for example: file to PI, jdbc to PI, IDOC to PI)...
The extrict theory says that you expose web services and you consume web services in on "Natural SOA"... Graphical example:
Natural SOA: ECC (consume the PI web services) --> PI (PI consume the ECC web services)---> ECC ... Maybe the similar standard technology that ECC use in natural SOA is: ABAP Proxy...
Remember that SOA is NOT a technical name that was born with PI, is an architecture where you can use with others Middlewares, BPMs, ETLs, customized code with web services (java, .net), etc...
This is just a recommendation: we know that when we implement something, we dont ever follow completely the standards or best practices, but those could help us a lot if we take it as reference or guide... as you are doing now asking for recommendations...
Good luck, i hope i have solved your doubt...
If you have doubts feel free to ask me,
Best regards,
Azael Navarro J.