They say that the 5G (but also 4G LTE) radiation could have effect on the blood that could make COVID-19 effects worse. It would be similar to the effect air pollution has on some diseases (making you more likely to catch them or die from them).
They base some claims on non peer-review studies and the studies they've done themselves were on a very small scale (and generally speaking, the effect of low-power radiowaves on the human body are complex to study), so it's more of a "there might be something there" and even if there is, it's unclear how strong the effect would be.