Many websites state that the digestive enzyme ptyalin cannot digest
starch when sugar or acid is also present in the mouth.  This would
imply that putting honey on your bread or adding vinegar to your
french fries would prevent the first stage of starch digestion.

Is there any scientific evidence for the above?

If it is true, surely we shouldn't be eating cookies, pastries, cakes,
most sandwiches, pies etc etc.  I know that starch is also digested by
amylase further down the gut, but surely we wouldn't have evolved
ptyalin in the saliva unless there was a significant benefit to
health.

Well, ptyalin in the saliva could break down starchy residues in your
mouth *after* you've eaten.
That would help preserve your teeth. That's a benefit :-)