"Transparency allows readers to judge..."

Methodological transparency.

It means the historian openly shows

which sources were used,

which sources were excluded,

why they were excluded,

how the conclusion was reached,

what contrary evidence exists.

Example.

Instead of writing

"Everyone supported the reform."

the historian writes

"This conclusion is based on 63 parliamentary speeches, 18 newspaper editorials, and 24 private letters. Five letters expressed strong opposition."

Now readers can evaluate whether the conclusion fairly reflects the evidence.

That is methodological transparency.