This is half-true at best.
As someone who's helped build FDA-approved genomic tests for cancer treatment, a huge proportion of cancers (in the US) are treated by sequencing and informing treatment based on sequence results. Especially among stages 3 and 4.
Diagnosis of early-stage (1 & 2) cancer via genomics is a difficult problem, because when the cancer is small, so is the amount of DNA it gives off. Liquid biopsy tests have a lot of trouble reliably distinguishing that small signal from noise, and it's not really cost-effective to test every single person for stage 3+ cancer via blood tests on a regular basis. Treatment of some early-stage cancers can suffer from the same problem, where it can be difficult to obtain a sufficient sample of cancer genetic material.
But the treatment landscape for late-stage cancers looks utterly different than it did 20 years ago. The idea that most stage 4 cancer is still treated in a "classical" manner in the US is patently false.
In some of the big cancers, sequencing is even a primary diagnostic:
* breast cancer: gene expression signatures are standard for determining the necessity of chemo
* colon: blood based sequencing for KRAS mutation is extremely common
* lung: matching kinase inhibitors to EGFR mutations is essential for therapy
* all solid tumors: tumor mutation burden is a companion diagnostic for the major immune checkpoint therapies that have revolutionized treatment over the past decade.
I know of almost no cancer that has not had massive changes in treatment, due to accelerated research possibly only through extensive use of genomics.