After reading the question and looking at the extremely well documented answer, citing sources from professional publications, it appears to me that questions about driving that are related directly to the health of the vehicle should be given serious consideration for being considered on-topic.
If the question were related to the legality of engine braking or asking if engine braking is safe for the passengers in the vehicle, then without a doubt it should be closed as off-topic.
In other words, questions that have nothing to do with the health or maintenance of the vehicle are and should be off-topic.
But this question, although an operational question, there was a problem directly related to potential risks to the health of the engine and drive-train, and any type of preventative maintenance, even operational preventative practices, should be considered on-topic here, IMHO.
The question was described very clearly, with exact speeds and RPMs and enough information for a professional mechanic to have a clear picture of how the driver is operating the vehicle and how that could affect maintenance costs in the future.
In addition, the answer that the question received is not one that offers just an opinion, something that can be easily debated. Instead, the answer is backed up with legitimate sources, which is one of the criteria for a great answer on a subjective question. It's clear the answerer did enough research to where I can be confindent in knowing that engine braking not only won't hurt my car, but it will save me time and money in brake jobs.
All in all, these types of questions could add a lot of value to the site. If questions people ask here can save you and I a lot of money through preventative practices, then this site will be more valuable than it would be if I can only ask questions here once I blow up my engine. ;) We want those people to come as well, but why not prevent disasters before they occur.