The purpose of necked-down bolts is so the shaft is more elastic, to act more like a spring to maintain a more constant clamping force on something like a head gasket, as parts change dimensions from thermal expansion. Since the camshaft towers have no gasket, that seems unneeded here, but perhaps they used them due to mixing aluminum and steel parts. Most head bolts don't need a neck-down since they are long enough to be elastic. Torque-to-yield bolts are torqued to a value, then marked and turned an exact angle from there (ex. 1/2 turn). That is because the turning torque can decrease once the bolt starts yielding, so if you kept trying to reach a higher torque value, you would just neck down the bolt and break it. This bolt is thus not a torque-to-yield assembly.

Perhaps M-B got reports of snapped bolts so changed to a stronger straight-shaft bolt. Personally, I would have gone to Ace Hardware and looked for a Grade 8 bolt (not sure they have in metric). 28 ft-lbf is nothing, almost 1 hand on a 3/8" socket wrench, so little worry about snapping a quality bolt. Insure you use 2 hands when torquing, keeping one pushing against the head of the torque wrench so you apply a pure couple load and not also a bending load like you would by just pulling on the handle, though unlikely that caused this bolt to snap.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans