Yes, with Rainboe on this one.
ZFT,
Unless your captain had calculated take-off performance from an intersection but actually commenced from closer to the downwind threshold (longer Take Off Run) then the take-off calculation is invalidated by ambling down the runway as you report.
There is always the possibility that it was done intentionally to check something.
Comments here and in other threads certainly suggest to me that, if we are going to do anything unusual which can be observed by passengers then the captain or first officer should mention it beforehand to reassure pax & CC.