It is not irrelevant. It is an open issue - since SCOTUS has NOT ruled on it. It is NOT directly addressed by Stevens ONE WAY OR THE OTHER.
HOWEVER, the fairly recent ruling in District of Columbia v. Heller [2nd Amendment - Right To Bear Arms] WAS decided on the meaning of the ORIGINAL wording and ORIGINAL intent of the Founders, since the Constituiton HAD NOT been amended to further clarify the meaning the following:
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
IF a challenge to the autopen is ever accepted by SCOTUS, it MIGHT likely be decided using the same criteria. What is the original meaning and original intent.
The case law you cited is irrelevant. The line-item veto case had nothing to do with what qualifies as a Presidential signature. You’re quoting nonbinding dicta in Stevens ruling ... and it doesn’t even make the point you wish it did.
The original intent was that the President sign a bill to confirm his consent to its passage. Obama consented, and confirmed it by authorized autopen signature. Non-issue.
Heller is also irrelevant to this matter.
SnakeDoc