Without a more complete rendition of the quotation (until I get home and can look it up), I'm hesitant to accept Stroebel's quote, as he might be speaking of M-synced flash (the setting used for magnesium-wire flashbulbs). Bulbs must be triggered before the shutter is opened to allow the burning reaction to get going.

That being said, I think that shutters are made with a little slop in the flash-triggering mechanism. The greatest opening force, applied through the shortest and most controlled lever arm, is applied to the blades to get them moving. At the end of the opening sequence the lever arm is lengthened (to provide more velocity) which means the opening force is much diminished, and the mechanism relies on the inertia of the blades to achieve the full-open position. I suspect (without evidence) that if the blades had much friction, as they might in a horizontal position, that the full open position would be slowed, even though the opening mechanism was at its fullest extension.

But even if this is so, it would only affect exposure at the widest aperture. For typical, stopped-down shots, the effect on the exposure would be none, none...more black.

And herewith I officially enter myself as a belligerent in this thread.