What kind of substrate is this?

This is following on from a post I made yesterday about replacing bathroom tiles. This is the ground floor, bathroom is an add on to whenever the previous owners were doing up the place years and years ago.

Just took up the corner of the most cracked tile expecting to find either a plywood or solid concrete substrate. Instead I have what feels like clay and you can see in the video I can scratch away at it. It is fairly tightly compacted, and feels solidish. The tiles have been there years with no issues. They just started to crack from when a tiler put down tiles out in the hall 2 years ago and was overly eager when taking up older hall tiles.