If you are earning just under ~$19/hr, then the employer is paying about ~$1.5/hr in taxes on that income, so not exactly a large chunk. The $19/hr is before the employee pays taxes. If you remove that, $19/hr is really more like $16/hr. 16/20.5 = ~80%, so the vast majority is going to the employee still.
This is only using federal taxes since state/local taxes vary, so that's an important limitation, but the sum of all state's taxes is about half of federal tax revenue and most likely comes primarily out of the employee's $19/hr rather than from the employer. Also non-monetary benefits are a thing, so if they, for example, cover health insurance cost or employer 401k matches, then the total compensation may be $1-3/hr more than the monetary pay. In total, that's a max of about $6/hour difference (more likely, about $2/hr-$4/hr) from the stated pay. Still short significantly short of just COLA adjustment from just 2008 being demanded even if you assumed payroll taxes didn't exist in 2008 and employees had no benefits then and high-end benefits now (not that you were arguing anything about that number; just wanted to give some context to the numbers and the demands).
Ultimately, I agree that its an unhelpful metric. The productivity of workers as a whole doesn't determine the value of specific workers and GDP can often count negative costs as a plus. Personally, I think the over-abundance of cars is a huge problem in our society, so if we based compensation on how much value is produced for society, most autoworkers might come out in the red. But if we wanted to use traditional economic measures of "value", then looking at the costs (excluding executive compensation) and revenue from car sales would be a more direct way to accomplish what they're trying to with that metric.
On the other hand, it provides some rough idea of how much we can afford. Only about half of the country is considered part of the workforce. Some of that is children who are being supported by that $19/hr the employee earns. But some are retired people who put in money over the prior 3 decades. Some goes to feeding children in poverty. Some are people with disabilities who need support. Some are people between jobs, but receiving unemployment insurance benefits (that they previously paid into). All of that has to come from the $73/hr. Some of that $73/hr goes to public programs that said workers can directly benefit from (things like roads, trails, parks, libraries, fire departments, medical research, funding the NLRB, etc). Without first subtracting all of those kinds of things from the $73/hr, you can't really use it to estimate what the average worker compensation should be.