As a molecular biologist, It just seemed like an obvious connection to me given the time frame. I have not googled the hypothesis but if geologists have noted years ago that the magnetic field was severely weakened during that geologic era, I would be surprised if no evolutionary biologist noted the connection. That said, I don’t see that mentioned as a hypothesis for the Cambrian explosion on wikipedia.

The magnetic field both causes and impacts the rate and most development as evidenced by magnetosomes in the fossil record and organisms today. Some species exhibit greater function in hypomagnetic conditions, and may have evolutionary impact when combined with known mutagenic effects of radiation exposure. We don't have known mechanism of action identified yet for the HMF (hypomagnetic field) theory and observations. Link to a review of speculations about causes and effects relating the magnetic field to the Cambrian explosion.

I’ve heard this theory before, but there’s no real evidence to support it besides it being a coincidence.

As mentioned elsewhere, life was aquatic back then and water is a pretty good radiation shield. Even most planktonic algae are mixed throughout the surface layer, which can be hundreds of meters deep, they don’t float at the surface for long