Isen workshop: adventures in batch production (or not...)

Why as a consumer should I be expected to question what the qualified expert who is building the frame tells me it can fit?

Because it's easier than trying to fix it after you've bought the wrong bike. I'm not defending Isen, but you wouldn't have got sucked in by their dubious marketing claims if you had a basic grasp of the dimensions at play. If you want fat tyres and narrow Q, something else has to give.