The input impedence will vary with audio frequency, and you might have a problem if the impedence of a nominally "high impedence" input goes too low somewhere in the audio frequency range. So their response, which I'd translate to "it doesn't drop below 5.3K Ohms anywhere in the audio frequency band" is reasonable. Two other manufacturers might well quote input impendences of "5.3K minimum" and just plain "5.3K" to indicate the exact same measurement.
That said, the 47K you quote for a Sony DAT feels like a more normal figure, and the 5.3K feels a little low. Whether it's actually too low depends on what you plug into it. You should check if the preamp or other box you intend to connect to the MT specifies a minimum impedence to which its output should be connected.