Well, that put me in my place. Consider me suitably chastised.
There's no mention of the phrase "client requirements" in RFC5321, so it's a little tricky to determine what you're actually referring to. The closest match I can imagine is probably section 3.2, "Client Initiation", which I'll quote in its entireity:
Once the server has sent the greeting (welcoming) message and the client has received it, the client normally sends the EHLO command to the server, indicating the client's identity. In addition to opening the session, use of EHLO indicates that the client is able to process service extensions and requests that the server provide a list of the extensions it supports. Older SMTP systems that are unable to support service extensions, and contemporary clients that do not require service extensions in the mail session being initiated, MAY use HELO instead of EHLO. Servers MUST NOT return the extended EHLO-style response to a HELO command. For a particular connection attempt, if the server returns a "command not recognized" response to EHLO, the client SHOULD be able to fall back and send HELO.
In the EHLO command, the host sending the command identifies itself; the command may be interpreted as saying "Hello, I am <domain>" (and, in the case of EHLO, "and I support service extension requests").
The parts of that which cover "what to send with the EHLO are:
the client normally sends the EHLO command to the server, indicating the client's identity.
That makes no mention of what "the client's identity" is, so that's not much help. You claim:
The client's FQDN is what is being sent. That the client's FQDN doesn't match the rDNS of the IP address it is connecting from is irrelevant. Clients can have multiple IP addresses, and multiple clients can have a single IP address. This is why the interpretation of the RFC being proposed in this thread is so non-sensical.
the command may be interpreted as saying "Hello, I am <domain>"
So it's got to be a <domain> that is sent, apparently. No mention of adherence to rDNS, let alone resolvability of the domain.
If I've referred to the wrong section, I'd appreciate a pointer to the correct section (they're numbered, for your convenience), along with relevant quotes and your interpretation of how exactly that requires the behaviour you're advocating.