being transgender is like being left handed in that it is an innate quality which cannot be changed even if it is suppressed, but there’s nothing in what he wrote here which is an argument for that implication.

There is nothing implied here. The parallels between the two is the explicit argument of the quote.

The apparent growth in the transgender population is similar to the apparent growth in the left-handed population. Transgender, gay, and left-handed people were all around before it was socially acceptable.

It was known and in the public consciousness long before it was tolerated, and people would persist in being transgender, gay, or left-handed despite it literally being a crime.

They were punished and oppressed, yet continued to exist. When it finally became more acceptable, each population appeared to explode by a sudden increase in people self-identifying and self-reporting.

But in each case it was always there. It's nothing new. A patient seeking medical transition was on TV in 1975. People just didn't notice because they are a relatively small minority, and because they were forced to hide from others - or worse - from themselves.