It looks like a pretty reasonable order of magnitude estimate to me. Energy arguments tend to be quite neat for that because if the efficiency is at all reasona

It looks like a pretty reasonable order of magnitude estimate to me. Energy arguments tend to be quite neat for that because if the efficiency is at all reasonable they constrain things well with fairly simple calculations. The antenna directionality is also reasonably well understood and characterised. The exact noise level discussed later on is probably where things get a bit more uncertain (but aren't directly needed to answer the question).