Yes, that is one scenario, but it remains the case that the Border Force can use their discretion, so that if someone gets stranded unexpectedly, or their luggage gets mishandled and sent to the baggage reclaim area. The BF agent can simply walk through the same checks as the online version (essentially checking a blacklist and crime records) and then let the traveller into the UK. Clearly if it's a few passengers then that is easy enough to do, but the reason for the ETA is more about bulk clearing people who mostly have no reason to be concerned about their bona fides.