inconsistently applied point
I can kind of see their point in this instance because the man in the parable is representing Christ.
I do not think that an inconsistently applied point or a point based on divers measures, divers weights, or double standards is valid. Is it really unscriptural to translate this word as "Sir" when addressed to a certain man who owned a vineyard?
Would Gail Riplinger and other KJV-only advocates condemn the KJV translators for translating this same Greek word as "Sir" when actually addressed to Jesus at John 4:11, 15, 19, 49?
At Revelation 7:14, the earlier pre-1611 English Bibles had "Lord" while the KJV revised it to "Sir."
Several of the early English Bibles such as Tyndale's, Coverdale's, Matthew's, Great, and Whittingham’s have the rendering "Sir" at Matthew 18:26.
At Joshua 5:14, there is the case of an appearance of God or the Lord Jesus Christ as a man [captain of the host of the LORD], and He receives worship from Joshua, but "Lord" is not capitalized in most present editions of the KJV although it was in many pre-1769 ones. It is capitalized in the 1560 Geneva Bible and in the NKJV.
Joshua 5:14 [my Lord--1560 Geneva, NKJV] [see Gen. 19:18]
my LORD (1679, 1771, 1772, 1773, 1778, 1783, 1788 Oxford) [1765 Cambridge] {1684, 1705 London} (1769 Edinburgh)
my Lord (1675, 1715, 1720, 1728, 1747, 1754, 1758, 1765, 1774, 1777 Oxford) [1629, 1638, 1683, 1743, 1747, 1756, 1760, 1767 Cambridge] {1611, 1614, 1616, 1634, 1640, 1660, 1672, 1711, 1735, 1741, 1747, 1750, 1767, 1795, 1879 London} (1638, 1722, 1766, 1787, 1791, 1793, 1842, 1858 Edinburgh) (1866 Glasgow) (1762 Dublin) (1782 Aitken) (CB) (1910 Collins)
my lord (
1769 Oxford, SRB) [1762,
1769 Cambridge, DKJB] {1760 London}