I understand.
It would make sense in so far as a straight, say 2p, on income tax, would be the cleanest way to raise revenue, and would unquestionably have been welcomed by the bond markets, despite being a clear manifesto breach.
I think what your friend is saying is, that therefore the OBR’s improved numbers were a consequence of the bond markets reacting favourably to the proposed income tax hike, before it was pulled/replaced.
The large bond investors are primarily concerned with inflation, and fiscal discipline (read headroom), through a combination of tax rises (and ideally, spending cuts), hence the markets have been relatively sanguine recently, given the former. They are naturally unkeen on uncertainty and flip flops, so I struggle to see how Reeves would have benefited from another u-turn.
For context, taking 5 year gilts, YTD they have traded at 3.85-4.6% approx, with today marked at 3.97%. Point being, they are volatile.
If there was a particular fiscal event, where Reeves needed to ‘capture’ that day’s value - for mark to market purposes - I am not aware of it, but your friend may know better.
Sorry for the windy reply!
Edit: I see the team have found the answer - chapeau!