His commitment to false theology demanded he reason it so...
"Robinson was considered a major force in shaping liberal Christian theology. Along with Harvard theologian Harvey Cox, he spearheaded the field of secular theology and, like William Barclay, he was a believer in universal salvation.[4]"
From his Wikipedia biography.
Theology and forensics are two different things; Robinson used the latter to show the dates of the NT books.
And a question;:
Why wasn’t dispensationalism (Darby, etc) taught before the 1880’s?