on the list of 'terms i keep seeing get substituted for one another that don't mean the same thing At All' are 'no less' and 'none the less.' they sound similar, right? almost the same.
but 'no less' is an originally somewhat slangy sentence-ending construction that adds emphasis, with an overtone of suggesting that the egregious thing being described is excessive and the listener is invited to be outraged. it's like 'at least,' but with different subtext.
developed from the fairly straightforward idiom of going 'no less' about an unreasonably high price, expanded out of that context and largely divorced from actual questions of measurable quantity.
when you say 'she did this after dragging me around for six hours, no less!' the emphasis is not being put on the sixness of the hours as the literal wording of the sentence might suggest, but on how stacking this objectionable thing she did afterward on top of those six hours is really too much.
if you say 'with my brother, no less!' there isn't even a number present, just the idea that stacking the involvement of the brother on the other transgression makes it even worse.
'none the less' on the other hand is more formal, and more or less synonymous with 'even so.'
when you say 'none the less' you're saying 'despite that thing just established, the other thing has not been reduced.' very literally 'no subtraction tho' or 'those don't cancel out.'
no metaphor or semantic drift here, really, it's just a straightforward verbal construction that people generally wouldn't make today, fossilized into modern speech as idiom.
they do not mean the same thing at all. they point in opposite directions; one adding drama to something being recounted and the other dismissing it.
i don't see this swap in writing all that often, but i hear it out loud with surprising frequency.
