It's a meta-analysis of studies estimating prevalence in general populations from sample data.
And right there in that sentence is one big clue that you can't simply present this data as if it were itself an estimate of the prevalence of transgender people amongst the population. From the abstract: "to compare findings across studies that used different methodologies, in different countries, and over different periods."
If the study that I cited is to be excluded from the discussion because it's from 9 years ago, then why would 27 studies from different countries, different time periods, and which use different methodologies, be
more valid? Can you even say that a single one of these studies is more recent than 2011?
To look at the actual study, rather than just the abstract:
The reported prevalence estimates are greatly affected by differences in methodology, and by variable definitions of transgender.
So the methodologies are important, absent the various definitions.
The electronic search included all papers published from 1966 through January 2015.
So the papers examined can go back a lot further than 2011. Do you think, perhaps, the changing acceptance of transgender people in society between 1066 and 2015 might affect the availability of gender affirmation surgery and people's willingness to out themselves by undergoing it?
As it is, the data actually goes back to 1953. If 2011 is deemed to be too long ago to be worthy of consideration, then why would 1953 be better? One study covers a period from 1958 to 1968.
Similarly, even ignoring the temporal aspect, why would data from Singapore be directly comparable to data from Iran and Spain?
And
not ignoring the temporal aspect
or the differences in population, why would data about the general population from 1968 in Sweden be directly comparable to data about veterans from 2011 in the US?
So, lets get on to methodologies. Compare and contrast:
Conron et al. analyzed data collected between 2007 and 2009 from the Massachusetts Behavioral Risk Factor Surveillance Study (MA-BRFSS).44 The survey was administered to 28,662 adults ages 18 to 64 years. Each participant was asked: “Some people describe themselves as transgender when the experience a different gender identity from their sex at birth. For example, a person born into a male body, but who feels female or lives as a woman. Do you consider yourself to be transgender?” There were 131 participants who responded ‘yes’ to that question, corresponding to a prevalence of 500/100,000 persons. The authors acknowledge that their estimate may have been affected by misclassification bias due to the broad scope of the question. Another limitation of the data is a lack of information on natal sex and gender identity.
Gates combined reports from the 2003 California LGBT Tobacco Survey, and the 2009 California Health Interview survey in order to estimate the number transgender adults in the general populaiton.16, 44 The 2009 California Health Interview Survey reported that 3.2% of adult participants were LGBT. Based on the 2003 California LGBT Tobacco Survey, 2.4% of LGBT persons self-identify as transgender. Using these considerations Gates further estimated that the prevalence of transgender people among California adults is 0.1% or 100 per 100,000.
Do those two papers sound like their data can be compared directly to get a true figure for how many transgender people there are in the general population?
And here's the kicker:
[...]the current communication should not be viewed as an attempt to obtain an average measure of transgender prevalence.
The analysis is about showing how differences in data collection and analysis methodologies can lead to wildly different results and that in order to collect meaningful data "future research should employ established formal methods of prevalence calculations".