Why did he use graphite as a filler? The filler is added partly so that the glue "self-shims" a space for itself between the pieces. It has to be strong and have it's own good bondability characteristics too. Silica is the usual choice.

Allowing the epoxy to gel before the pieces are set together works well if he wants to skip the filler. As long as the excess epoxy still flows reasonably well when the pieces are clamped, it'll set up to 100% strength. Whether he uses filler or skips it, clamping has to be light enough that the epoxy isn't squeezed out excessivley. If the pieces don't slip against each other somewhat when they're pushed, it's probably clamped too tightly.