

The “TM SGNL PIN” message is displayed on an unofficial — and less secure — version of Signal created by a company called TeleMessage, which makes clones of popular messaging apps, but enables the ability to archive messages
TeleMessage is an Israeli software company based in Petah Tikva, Israel. Founded in 1999 by Guy Levit and Gil Shapira, it provides secure enterprise messaging, mobile communications archiving and high-volume text messaging services.[1]
https://en.wikipedia.org/wiki/TeleMessage
Let that sink in
Signal is lacking a certain feature “Archiving messages” . This company used Signal’s open source code to add this feature and offer the whole thing as a “Signal plus” product.
Assuming best intentions, Waltz wanted Signal but also wanted the ability to archive messages so he opted for the software provided by this company instead of the original.