I'd like to think that was accounted for in the analysis.
We can compare children infected last year when other strains were dominant, with children infected this year in UK (likely to get the Delta variant).
Anything else is one wolf deciding to eat two lambs. Also, if you enslave almost half the population you definitely don't have a democracy anymore -- unless your slaves are allowed and able to vote and participate in public discourse like anyone else, which likely would not quite be slavery anymore.
Their point was a bit more subtle, it meant that EU carriers would effectively be penalized relative to non-EU carriers operating in the EU, because other agreements precluded applying the tax to those (non-EU) carriers.
Means they need another tool.
The exception for private jets seems hard to defend though, whether business or pleasure.