The announcement for v4.51 does state that the PX1000 is no longer supported:
This release has many other changes which will not be readily apparent to you as end users. Mostly, such changes are code optimizations and fixes for less-common bugs. However, there is one important caveat: v4.5x firmware does not support the PX1000 module, which is an old pre-Apex module that has not been sold for many years. Other legacy/pre-Apex modules such as DC8, DC4, DC4HD, & LunarSim continue to be supported in v4.5x firmware.
The same statement is also in the announcement for v4.52. Granted, it was no longer noted in the subsequent v4.53 release announcement. The sticky you linked does say this at the end:
Note: The Probe Expansion option is not present in the Apex 2016 model, in the Apex Jr, or in Apex Classic/Gold/Lite models which are running firmware versions higher than 4.40.
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