I strongly agree with this point, and don't buy the blanket rationale that speed trumps everything in a startup.
I couldn't find its meaning anywhere, what does blanket rationale mean?
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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.