The wavelengths are too large for LIGO. The wave measurements were correlated with data collected from many pulsars.

https://arstechnica.com/science/2023/06/nanograv-picks-up-si...

Thank you for this article link, I found it much more informative than the parent article. The video at the bottom was well made.

It's my understanding that LIGO is too small to be able to detect such waves, hence the use of pulsar timings.

As others pointed out, LIGO is too small compared to the wavelength of the GW, but even if LIGO was extremely long, the technical noises (seismic, control system noises, gravity gradient noise, etc.) at such low frequencies are extremely high for ground-based detectors