The theoretical density they give neglects some overhead. If you divide the area of a chip by the size of a transistor, you should get 100 billion transistors on a chip, but in fact you can only get about 1 billion. The rest is overhead: wiring, power, isolation, etc. Probably a similar overhead will apply to memristors.
Also, in the 5+ years it takes to make them practical and reliable, transistors will make progress too. So it's unfair to compare the theoretical density of a new technology to the achieved density of an existing technology.
Also, in the 5+ years it takes to make them practical and reliable, transistors will make progress too. So it's unfair to compare the theoretical density of a new technology to the achieved density of an existing technology.