Thierry Martinez quite simply said that the living arrangements was the reason the deal collapsed. A flat was promised which they were happy with then the club decided to change the living arrangement which was the deal breaker. And he's also quoted as saying it was him who didn't think it was right for the young teen at the time and not his parents. The full quotes are below. So not getting the flat was the deal breaker and everyone can read the full quotes below with link provided.
What the player said later is just PR because he was quite obviously close to signing for us and even agreed most things with the club via his agent.
Thierry Martinez: Martinez told So Foot: "An intermediary met with us, but between the project she presented us and what was actually going to happen.
"A flat was promised, but that changed to a host family with only six return trips for his parents.
"When you uproot a player at such a young age, that isn’t enough. I wanted Dayot to have a stable emotional balance off the pitch."
https://www.mirror.co.uk/sport/football/transfer-news/dayot-upamecano-broken-promises-saw-13968638
The bit about the flat was reported by the English Journos too, long before Thierry Martinez mentioned anything.