Yeah, that's pretty old, and if you asked planetary scientists a decade or two ago most would have doubted terrestrial planets could have commonly formed that early. But data are data, and we're seeing that this actually does happen at least some of the time.
We're also seeing that
there doesn't seem to be a very strong dependence on terrestrial planet frequency with star metallicity, although there is a strong dependence for giants. What this tells us is probably that the dominant mechanism for giant formation is core accretion, and in the early universe the core accretion was less likely to build embryos large enough (or quickly enough) to become giants. So it might make some sense to see old terrestrial planets as "failed giants".
Plus it was 1000 degrees warmer back then. Fourteen per cent of the intergalactic gas was helium 12 billion years ago, absorbing the intense radiation from active galaxies, losing electrons in the process until it was ionized completely, and since it had no more electrons to lose, the radiation simply passed through the gas without heating it. At this point the cooling effect of expansion took over.
Whoa, hold on. We can't just say that it was hotter -- it depends on the context. The temperature of the intergalactic medium is very different from the temperature of everything else, similar to how the temperature of the Sun's corona is orders of magnitude hotter than the temperature of the surface. The density matters, and the mechanism by which heating (and cooling) occurs.
In the context of planet formation, we should look at the ISM temperatures. While it's easy to heat diffuse gas by photoionization, it's not so easy to heat a dusty interstellar medium or molecular cloud. 11 billion years ago corresponds to a cosmic redshift of z~2.5, and we see that the ISM back then was typically quite cool -- tens of Kelvins. Plenty cool enough for star and planet formation.