A blanket statement is a generalization – something that covers everything, like a metaphorical blanket. There is an implication when using “blanket statement” that it may be an over-generalization. For example, “Snow is cold” is a fact; “Nobody who lives here can drive safely in the snow” is a blanket statement, and most likely not completely accurate.

A rationale is a justification, a reason to make a decision. For instance, I might put chains on my tires, and my rationale might be that heavy snowfall is predicted.

So, to put the two ideas together, a blanket rationale is a justification that may be based on an overgeneralization. In your example, the speaker believes that many startup companies justify decisions based solely on speed, and implies this is a flawed argument.