Yeah, those are the nrf51x line, this is the nrf52x line. It seems from the outside at least that Nordic is pushing the nrf52x hard even in price conscious designs. Even the micro::bit was respun for a v2 with a nrf52x also (including it's M4). Albeit I don't have much direct data on that.
Yes, the 51 is really old by now. The 52 line extends down as far as the tiny nRF52805.
The 51822 is not nearly as fun to work with as the 52s, e.g. current Apache Mynewt with NimBLE doesn't even fit into the 51's tiny flash, the 51 doesn't have systick, and so on.
