Well I didn't found anything regarding this on GitHub. And to be honest I only find some german language articles about this speaking of an announcement that signal don't likes the idea enabling Chats with WhatsApp... so I assume this comes from Twitter.
Like I said turn it of by default so that only people who activity made the decision can be chat with WhatsApp. Also showing a hint in the chat that this account is not on a signal server should also not be that hard to implement.
I don't get it... "D" is a complete different character than "d" is.
It's like wondering why "file1" is not opened when I typed in "file2".