You followed a link to mandarinretreat.com, so here's the short version.
Where the retreat is now
Mandarin Retreat started in 2022 as weekends away in the UK where everyone spoke only Chinese, in places like Chester and Scarborough. Karl Baker, who now makes Mandarin Mosaic, was one of its co-founders.
The retreat is now run by I'm Learning Mandarin. For dates and bookings, see their retreat page.
Keeping it going at home
A weekend of speaking only Chinese teaches you two things fast: how much more you understand than you thought, and how often you still freeze when it's your turn to talk. Karl's two projects since then each take on one of those.
To understand more: Mandarin Mosaic
Mandarin Mosaic teaches through real sentences, each with one new word, so you're always reading something you can almost understand. Spaced repetition brings words back just before you'd forget them. It also has HSK lessons, a graded Chinese news reader, audio for every word and Anki import. It's free to start on iPhone and Android.
Will Hart came on one of the early retreats. He and Karl have since built Chunking Chinese, a speaking trainer that runs in your browser. It teaches the ready-made chunks native speakers build sentences from, then has you say whole sentences out loud, and marks the tone and sound of each character. Try it free for 7 days.