The clue is in the Latency
If we look at the Financial Trading Systems where this sort of OS is used, a guaranteed latency is the name of the game.
These systems broadcast data to tens if not hundreds of clients. The AMQP message queueing software gives that sort of guaranteed low latency messaging these clients demand.
I find the comparison with other messaging systems rather funny.
Websphere MQ? Yes but only if you use the Low Latency variant.
MSMQ? Are you having a laugh?
JMS? Well, OOTB JMS is quite slow but you can use lighter weight transports so probably Yes.
Once you get down to guaranteed latencies in the low microsecond range, pretty well everything can be classed as an RTOS.
The trouble with using a general purpose OS (eg Linux, Unix or Windows) as an RTOS is that in many cases other non Real Time applications are loaded onto RT systems. When that happens, they don't become RT any more.
Then we get to RT on VM's (apart from ones that get very close to the Hardware such as z/OS & IBM pSeries LPARS) and in the Cloud. For that, all I can say is, are you having a laugh? However there is no doubt that in time that will happen.