English is not my first language but I understand the term. The quote says "reduced but detectable levels 4 weeks after"... this is true but this "reduced leves" will continue after days/weeks until reach 0. I dont have time now to provide some links but there is a lot of information on this. You can even ask probiotic makers/sellers and they recognize this fact. No one probiotic will remain in your gut.
However, do probiotics themselves attach to intestinal
cells and thus proceed to colonize the gut? In vitro
studies using tissue cultures suggest that the answer is yes and that probiotics do (37–39) interfere with the adherence of pathogens, such as Salmonella typhimurium, to Caco-2 cells (40). However, is this also true in vivo? Currently available evidence suggests that it is not. For instance, the recovery rate of an antibiotic-resistant strain of Bifidobacterium in the feces was determined after it was administered to human volunteers (41). The recovery rate was 29.7% ± 6.0% of the ingested dose, which is consistent with the percentage survival during probiotic passage through the gastrointestinal tract (see above). WHEN ADMINISTRATION OF THIS STRAIN WAS STOPPED, IT WAS NO LONGER RECOVERED IN THE FECES:
"Upon cessation of daily supplements, the gut microflora will generally return to its normal state in about the same amount of time. So regular consumption, particularly in periods of stress like athletic competition ... examinations or extended travel, seems appropriate."