I'm not speculating at all. The article has the data to back the facts : house prices went down in Berlin.
Yes, homes can go up on speculation alone, but this is a bubble that will pop. In North America, rising rents act as a backstop that prevent the bubble from popping, because of guaranteed rental incomes that protect you.
It's not possible to create an insane level of supply for appartment rentals. There's two reasons for this. Firstly, there's the obvious limit to how much you can build within a reasonable commute time with reasonable infrastructure at a good quality of life. Secondly, studies have shown that 10% increases in supply lead to 1% decreases in price, more or less. Coupled with 4% rent increases over inflation every year, it's just not feasible to rely on additional supply.
In the real world, free markets lead to rent increasing right up until the level where too many people are in poverty due to high rent. That's because the natural increase in rent is simply too high to be counterbalanced by supply.
That's why from the worst planned cities to the very densest like Hong Kong, housing stays more or less as expensive as it can be without people moving out due to poverty.
The only solution is to make real estate a barely profitable or depreciating asset. This is done here by aggressive rent control. As renting stops being so affordable, apartments are sold, homeownership increases, and renting becomes something you do to prevent depreciation, not to make profit.
Crucially though, here, building more apartments stays profitable, because they can go to homeowners or to people who buy housing as a stable asset (and then rent it out). Coupled with public housing, you can get the best possible trade-off.