The fact is, thousands, if not billions, have been killed by drone attacks.
Routan, if there is no basis at all for the 'tens of thousands' possibility, if it's clearly false, it's worth criticizing its inclusion - and inferring that you might not have a good understanding of the facts because you think it is possible. It's not a semantic argument, it's a fair one on the substance - you gave him the ammunition for it.
You can criticize him for ONLY concentrating on the issue of your 'if not tens of thousands' phrase while pretending that the 'mere' thousands are so small as not worth worrying about, but your response on the issue of that phrase IMO is not reasonable - unless you can offer any evidence that there's some reasonable chance - I'd take 10%, maybe even less, but not effectively zero - that it's correct, you should acknowledge the phrase was an error and move on to discussing the real number.
On the other hand, his knowledge of the English language is wrong when it comes to numbers.
One might think, as he says, that "tens of thousands" means at least 20 thousand, because tens of thousand is plural, meaning at least two of 'ten thousand', but that's not the accepted definition. In fact, "hundreds" starts at 101, "thousands" starts at 1001, and "tens of thousands" would start at 10,001.