It's not a realistic danger and just fear mongering. I'm not sure why people on HN feel the need go after Signal so hard. I do think criticism is important (and Signal definitely deserves plenty) but these types of criticisms are off base and not specific to Signal, nor are they that relevant (kinda how people post on Signal's tweets about Iran complaining about lack of usernames. Not the time nor place).

It isn't meaningfully different from saying that Google/Apple can pretend to put the real App in the App Store but replace it with one that has a backdoor. This is entirely possible. But also the risk of this is extremely high and people do decompile apps like Signal, WhatsApp, and Telegram (albeit this can only go so far). These are all high profile and highly scrutinized apps. It is just fear mongering.

Before any backdooring purposes there is probably some marketing/analytics reasons, keys, OTF updates etc...