I'm pretty sure that by HTTP client he meant rendering engines. And in this analogy the data integrity is "whatever crap I have to send to the client to get the expected output on all client machines". So it's somewhat fair to say that if you have wildly divergent interpretations of HTML rendering then you have a data integrity problem, since you can't guarantee that the same data will be interpreted the same way by all clients. None of that has to do with HTTP the protocol though... so I'm confused as to why HTTP the protocol was mentioned instead of HTML and rendering engines.

I can't tell if you meant for this statement to be as insulting to Patick as it seems at first glance. If you did, I will point out that an alternative outlook is to realize that as Patrick is a very smart guy you may have simply misunderstood, misinterpreted, or misapplied his analogy.