Yeah, I noticed the same--there's nothing red on the desktop site, but when I opened the mobile app, it highlighted the "Accept or Decline" thing in the inbox in red, which gave me pause and had me Google for this thread.
On desktop, it does say this in the recently-redesigned message screen:
"Respond to the next inquiry within 24 hours to maintain your response rate."
Sounds like a response to the guest is sufficient, but it would be nice to know for sure. (They messaged me for a discount; they don't qualify to IB, but I don't want to outright decline them just yet, as their behavior hasn't crossed any line where I wouldn't deny them a stay.)