Torah Temimah cites Meilah 8b.
“It is a chatas” — this teaches that the squeezing-out (mitzuy) of the remaining blood of the bird-chatas is indispensable; without it the offering does not take effect.
Diyuk of the superfluous “hu” — marks mitzuy of the residual blood as me’akev.
If the kohen fails to press the leftover blood against the altar’s foundation, the bird-chatas is invalid. Mitzuy is not optional cleanup; it is part of the avodah itself and is indispensable for the offering to take effect.