The internet is 100% a series of tubes. This is how I was taught to think about networking in terms of bandwidth and throughput and routing since the early days when we were laying out what would become 'dark fiber'.
"A series of tubes" was the same kind of political character assassination that led to Howard Dean getting ridiculed for his infamous scream. He butchered the sentence. Fair. But Stevens should be ridiculed for parroting a tech industry lobby stance about net neutrality, not for the series of tubes metaphor.
You are probably too young to remember that the dominant metaphor for the internet in 1990s politics was "the information superhighway." It was easy to think of the web as "driving" browsers to visit web "sites", with slow bandwidth being analogous to being caught in traffic. But the internet is closer to water, gas, and electricity than roads. Concepts like bandwidth and throughput are closer to how they play out in infrastructure policy for various things with tubes, versus cars and roads. Do you think he's wrong and that the internet is closer to "a big truck" versus "a series of tubes"?
The issue being debated was net neutrality and bandwidth, including specifics about who pays for what and the downstream second-order consequences of various policies. He was parroting some line from some telecom lobbyist, but the point the lobbyist was trying to make through Stevens was about how if certain policies about who pays for bandwidth were adopted, it could disincentivize some things at the Tier 1/2 layer that could increase transport costs at the Tier 2/3 layer that impacts ordinary people's bandwidth.