I’ve heard it said that Scrivener’s GNT is just a back translation of the AV; is this true?
No. The fullest and final representation of the TR was produced in 1611 by the translators of the Authorised Version through their textual choices. However, they did not produce a GNT showing their text. Scrivener went through the AV, comparing it with the TRs used by the translation committee and attempted to determine which readings they used where they differed from Beza’s 1598. He didn’t back translate, but used the AV to guide him in picking Greek readings.
He published multiple editions with the final being his 1894. This is the TR currently published by the Trinitarian Bible Society. tbsbibles.org