Well, not necessarily. I have heard that some people join the Mile High Club while in their seats, "sleeping" under a blanket. And remember that story of how Hatfield was supposed to have stood waist-deep in a pond while mixing anthrax into envelopes in a drybox with glove holes under water? Couldn't a terrorist, after practice, get his bomb together while lying in the dark in a first class seat with his eyes closed? If his knees are up in those recliners, he would have plenty of room under the knees to move without being detected.
I suppose it could be done, however overall very difficult, given the darkness and chance of being caught by the crew or an adjacent passenger. Richard Reid didn't even have enough time to set off his shoe bomb while sitting before being caught, let alone another scumbag terrorist trying to assemble an entire bomb. The bathroom would provide light and complete privacy and would most likely be the site to assemble any bomb, Heaven forbid.