And he's right, up to a point, because Dumbledore borrowed the Invisibility Cloak and hung onto it for months when he knew the Potters were in danger (although Severus may not know that). It might well not have helped them even if Albus had given it back - but the fact that he held on to it shows he wasn't giving their safety his full attention, to put it mildly.

Very good. sad but seemly true.