It has changed. Long story - google "Lille Loophole" for the full details.
Basically, Belgium started objecting to the UK Border Agency insisting on checking passports of Brussels-Lille passengers, as this is against the Schengen agreement - at one point allegedly threatening immigration officers with arrest. This resulted in a situation where Lille passengers didn't have to show ID, so anyone wanting to enter the UK illegally could quite easily buy a Brussels-Lille ticket, travel to Lille, and stay one, holed up in the café bar or similar.
Once this was publicised, there was a predictable response in the UK, and incoming immigration checks were introduced at St Pancras for Brussels arrivals.
The eventual solution was to have Brussels-Lille passengers confined to a single, guarded carriage, aligned with a dedicated Schengen section of the Eurostar platform at Lille.