In order to understand "false pretender" one needs to understand the historical use of the word "pretender" as in " a pretender to the throne". To be a "pretender" does not mean one is necessarily illegitimate. A "pretender to the throne" could be simply "one who claims the throne or the title". A "pretender" could be legitimate royalty.

Therefore, a "false pretender" is a claimant who is not genuine, not legitimate. This all gets confusing because the word "pretender" has a different connotation today.

He was a pretender to the throne of the UK and he is known as the "Young" pretender to distinguish him from his father known as the "Old" pretender.

pretender

Be true to yourself and don't be such a pretender.

He's a Pretender was created in 1986.