Always conjecture as to where 'the' manoeuvre starts, but it is perfectly acceptable to complete a flypast at 100ft and ease up into the next manoeuvre even if it is a loop. It therefore does not follow that the loop was started from 100ft.
That said, the 'easing up' into the manoeuvre clearly has to be allowed for and not flown at maximum performance. The slight loss of energy in this 'easing up' also has to be catered for. Are we really expecting a displaying aircraft to climb up from a perfectly safe 100ft flypast to achieve 500ft, level off, then commence a loop?
Similar when you have to take off from a runway within the 230 line - you ease into the display complying with the AoB v height limitations; you're therefore not guilty of breaking the 230 line limitation.