Generally, the longer knotweed is left, the more costly and complex its eradication. I've seen no evidence of diagnostic test-pits here that would indicate the investigation of rhizomatous material, but that is not to say they haven't been carried out. A regime of glyphosate-spray and secure movement of material within site may need the addition of soil excavation and sieving/sorting professionally, along with further treatments and active monitoring over many years.
This is not to say that knotweed is uneradicable: it can indeed be completely eradicated...but note that it reputedly has been found to be viable after a dormancy of over 20 years.