I thought UK rules were bad

Sounds like they created a legal tenancy either accidentally or through exchange of money. Real estate agents are relatively expensive in Germany and often property owners skip due diligence processes they should really be following at their peril. I knew of someone who decided on just references over full background check...ended up with a family where it turned out dad had served time for murder and refused to pay 1 month after moving in. Unsurprisingly nobody wanted to go round for doorstep negotiations 😁 and of course it was too late to enlist an agent to support the process. Took 2 years to get them out and I seem to remember a cash incentive was involved. On balance the German system is far superior for stable rental market though, allowing it to operate as a genuinely viable alternative to property ownership (both for tenants and for landlords via tax system).