He says "By picking 31 bytes I ensured the sensitive information leakage would be negligible." but if you don't save/inspect other than verifying it's not 31x0, I don't see the problem?
If the first 31 bytes are not the same, then the server is likely generating a new session id, which is a bug but probably not leaking any information.
If the first 31 bytes are the same, then either the server is padding the session id with zeros (which, while buggy, would not be an information leak) or the server is sending back some uncleared memory. These cases could be checked by seeing if the padding is zero (which would have at least a 0.4% false negative rate with just the one byte) but the zgrab test in the article doesn't bother distinguishing them and just assumes any extra data may be an information leak.
EDIT: Re-reading RFC 5077 again, I'm not certain that the server is prohibited from sending back a new session id. If it is allowed to then the second paragraph above doesn't indicate a bug, just a rejection of the session ticket.
Never underestimate the ability of a company to suddenly invoke the CFAA in the USA (equivalent laws are available elsewhere). IANAL
In this case, I'd like to publicly applaud F5 for being so cool about it. Kudos, F5!