It's actually not super stupid fundamentally. Marx did have a theory about the nature of work, and what is considered a meaningful life. He argues that it's only the production of... well... things... that give life meaning. It's important to feel, emotionally, connected to the things that you produce. We of course, in the modern world think this is silly. But he was writing well before industrialism had fully transformed society. The society in which Marx lived and worked still drew upon values that came from the agrarian (and subsistence farm) world. So to him and his contemporaries it made a lot of sense.
His mistake was to run with this and stick it into his economic theory. Just because only the production of things is fulfilling and gives us emotional satisfaction/meaning doesn't mean that, economically only the production of things is valuable. He has conflated two different kinds of value. This is straight up an embarrassing mistake and he should feel bad about it.
