By every way that I can think to define “mass production”, I think that the PinePhone got there before the Librem 5, which isn’t surprising considering the design choices that PINE64 made compared to Purism’s design decisions.
If we define “mass production” as when a product has its full functionality available with only minor bugs, then neither the PinePhone nor the Librem 5 are at mass production, but the PinePhone is definitely closer to that state than the Librem 5.
If we are going to call the PinePhone “beta”, then we also have to call Evergreen “beta” or maybe “alpha”, considering the lack of FCC/CE certification and all the software dev work that is still required for things like the cameras, GNSS, OpenPGP, suspend-to-RAM, system wakeup by the cellular modem and WiFi, etc.
If we define mass production as when shipping a large number of the devices, then the PinePhone started mass production in June 2020. Maybe you can argue that Librem 5 started mass production in November 2020, but I don’t think 250 devices per month is a large enough number for that definition. At this point the PinePhone has shipped 20k - 30k units, whereas the Librem 5 is around 1200.
Another definition could be when the hardware stops changing. By that definition, I would say that the PinePhone reached mass production with v1.2b in October 2020. At this point we don’t know for sure if the Librem 5 will have any further hardware changes, but it appears likely that there won’t be any changes to v.1.0.2 of the schematics, which were released in November 2020.