This is absolutely incorrect, unless you are being exceedingly facetious about the scientific process. There have been some truly remarkable advancements that span the gamut from simple behavioral changes like reduced calorie intake to advances in knowledge of certain enzymes and other compounds that have yielded statistically significant results - published results of just these two examples have yielded up to 50% increase in longevity in various animals with the latter leading to accelerated FDA approvals for human trials.
I think you're attempting to split hairs by quantifying two necessarily qualitative terms which really have a common understanding to be how long people live on average. And in doing so, you commit the same error you highlight: surely you're not suggesting that longevity decreased on average between 1920-2000? Even if you hold fast the true importance of lexical disambiguation, most would disagree on straw man grounds.
Where I do somewhat agree or at least am very open to consider is that some other force is at play that we don't yet understand. The fairly recent theory of hormosis presents some profound philosophical arguments that cannot be ignored. Life as we know it may have evolved to live less than its full potential for reasons we have yet to discover - perhaps highly interdependent relationships between co-evolved cooporative cells can only resist entropy for so long. If this is the case, then this sort of predefined balance/order would mean gains are offset by newly emerging losses - an example would be the massive increase in cancer occurrence that has mirrored increases in life expectancy, moreso when this broad category of diseases is considered by its generic definition of "uncontrolled cellular growth".
So there's certainly room for reasonable skepticism and it should shape our expectations accordingly. But that shouldn't mean the end of these pursuits nor the hope for what they could yield. And prudent scientific observation should at least suggest that greater longevity is possible. I recently read an interesting article about jellyfish and Hydra, which go through various developmental stages that actually come full circle and leading some to theorize that these somewhat basic life forms may actually be immortal. There's also tardigrades, which can live in the most extreme environments imaginable, including volcanic vents and even celestial bodies traveling through space! They've experimentally proven all our understanding of life wrong and that's only in the past few decades. Kurzweil is overly optimistic about these things because I suppose there's a degree of fear and insecurity there - he's likely predicting accelerated time lines because that's a narrative he can be part of and wants to believe that to be likely for obvious reasons. But I don't think he's wrong and fully admire anyone promoting a hopeful and tenacious outlook on life instead of the increasing opposite trend of disillusionment and cynicism - but that's an entirely different discussion.