I understand the question.
Whether or not we are able to pinpoint the precise mechanism by which the mismatch causes trouble, it does seem pretty clear that it can cause trouble which is what the OP wanted to know.
It's not likely to blow up a gun, but it can cause problems which are serious enough that one would think any reasonable person would avoid them if possible.
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...primers don't pop out until pressures are higher than that.
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I think that's why so many people get away with the mismatch. What the mismatch does is set the stage for another factor, one that would normally be totally harmless, to push things past the breaking point. Sweeney's experiences in warm weather support that explanation.
And, it's worth noting that even if, for the sake of argument, we accept that 77Kpsi is not sufficient pressure to cause blown primers, in and of itself, it is still 40% higher the SAAMI maximum peak pressure specification for .223 Remington. That's well above even proof pressure.