Sorry, I should have used more words and written more clearly in that sentence.

It should have read:

Not having the end manifold bolt fasteners thread into bolt holes in the aluminum cylinder head at 90 degree square would be an improvement.

If the end of exhaust manifold bolt holes
and their mating threaded holes in the aluminum metal of the cylinder head
were drilled and tapped at some degree of angle outward,
and the hole in the manifold made oval shaped
(or simply bigger diameter)
the thermal expansion of the exhaust manifold would not result so quickly in
the cast iron of the manifold eventually contacting the bolt threads in a “shearing” type collision
similar to how sheets of metal are ripped in two by metal shop “Shearing Machines”.

“Avoid exceeding maximum allowable shear stress levels to avoid metal failure”

is the modern advice from
3 dimensional principal stress fracture mechanics.

100 years ago engineers thought you could
pull on a bolt too much (tensile stress)
or
push on a bolt too much (compressive stress)

Today it is realized that either
“pure tensile stress“
or
“pure compressive stress”
inherently also creates SHEAR STRESSES inside materials
and importantly these internally created shear stresses
are inclined at various angles
that look “cone shaped” in 3 dimensional view.

When one of these “cones of shear stress distribution”
reaches a certain level
cracks begin growing in size
and the material eventually splits in two.

Too high a “principal” shear stress in one small region
is the “true” enemy.

Three dimensional matrix principal stress analysis
....is the scientific “slang” for this

Works for rocks as well as metals