Unless you are suggesting that one might install concealed cables in a way which does not comply with the regulations, then surely "like any other cables" can only mean in compliance?
No, because for an accessory to create a zone for cables those cables have to be connected to it.
If you need a concealed cable <50mm deep and there isn't "naturally" an accessory to create a zone for it then earthed mechanical protection is the way to go.
Your suggestion would be compliant if the box contained a joint in the cable.
[FORESTALL]I am not responsible for any facts here which you find displeasing.[/FORESTALL]
Unless that was perhaps a very long while ago, then, as has been implied, I don't think that BS7671 would agree ...
522.6.202 of BS7671_2018 said:
Where the cable is connected to a point, accessory or switchgear on any surface of the wall or partition, the cable may be installed in a zone either horizontally or vertically, to the point, accessory or switchgear.
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