I was curious about that claim, so I did a bit of research and made a "back of the envelope" calculation. According to Wikipedia [1], the angular resolution of a telescope is proportional to <wavelength of light>/<diameter>. Starlink seems to operate from 10 to 40GHz, so assume the hardware has a 50% design margin so is capable of reaching 60GHz. Visible light has a frequency of 400 to 800THz, so take the middle of 600THz. Using those figures, it come out that the Starlink satellites, used as a telescope, would have the same angular resolution as a 1.3m visible light telescope. Is it enough to resolve exoplanets ? I'm not sure but I think not. Hubble is 6m and can apparently resolve some exoplanets so it's not too far off. Some launches to GEO would boost the resolution to the equivalent of 8.4m in visible. (IANAA)