Most likely you're right. It could be a bit of sensationalist or sloppy journalism. In the first paragraph they just mention that the research was done in Praga. They should have really specified if it was done only in certain council wards. Still a shock even if in a ward there's 1 in 4 flats without toilets.
According to data from two years ago 92 percent of urban housing in Praga was established before the war, many of them still in the nineteenth century. 40 percent of them does not have any bathroom, 26 percent do not even have a toilet.
Still it seems to be getting better there. It was a bit of a no go zone in the 90s and now the gentrification has started and it's all a bit bo-ho there with the Soho etc. Lots of new developments. Perhaps it'll become like the East End / Docklands in the UK.