I travelled London-Brussels and back in recent days. I bought the tickets a few months ago and got the ticket PDFs with no request for advance passport info.
I did get an e-mail not long before I left London, which turned out to be telling me that it was _essential_ that I gave them that information before travel. However, I don't always keep up to date with my e-mails prior to travelling, and when travelling (and why should Eurostar assume I do?!). Having got the tickets safely, I didn't consider giving them any extra info before travelling; indeed, I doubt I'd have bothered even if I had been up to date with my e-mails!
My check-ins at both ends were as unremarkable as always (both ticket scans at the gates, and passport checks).
As some of us assumed, there is and can be nothing retrospective about the API requests.
I realise of course that any future ticket purchases will require filling in that extra info before the tickets will be issued. But since the ticket check and the passport checks are separate, and at no stage do you ever have to show both together, I'm not sure what extra assurance this system brings. Or is it the case that the passport booths will check your passport against not only their lists of wanted people (etc etc), but will also check them against the list they'll have of the APIs of that day's travellers? I'm a bit unclear how this will work. Surely you could still buy a ticket in someone else's name (albeit needing to give relevant passport details to match that name), but use a passport in another name when you travel?
And what if you have two valid passports, and quote one for the API and use the other to travel? Would the system pick up on that? So much of the system of checks seems to be based on theatre and deterrence rather than guaranteed security.
Having said all that, on my return to London the other night, on the last train from Brussels, as everyone was being sent down the travellator near the front of the train to access the arrivals area, an official announced to people reaching the lower level that they'd need to get their passports out for inspection, since there was an arrivals check. I've never had that happen to me since the early days of Eurostar. Looking along the queues moving slowly towards a handful of what seemed to be manual and rather basic passport booths set up at the far end of that area, I noticed that the check was simply a quick glance at the passport, and at the passenger, and nothing more. But by the time I was halfway along, the people staffing the booths stepped down and told everyone the checks were over, and the queues speeded up. I assumed that if they were looking for someone specific, then they'd found them. Interestingly, the train had slowed right down a couple of times between the tunnel and London, and came to a complete stop around Ashford for a brief period; maybe that was to allow time for the reception to be set up and staffed at St P? This was a service ex-Amsterdam, via Rotterdam and Lille as well as Brussels, so perhaps carrying intra-Schengen passengers leading to suspicious that there was an intra-Schengen passenger who hadn't left the train?