The post doesn't say "imperative", it just differentiates between defining pipeline steps and defining the logic within a step.

...also, TCL? I haven't used it for ops, but my memory of tcl/tk is extremely negative.

...also also: a core part of a build, CI, or, CD pipeline is almost always invoking binaries to run a command. That's why shell scripts are so ubiquitous in pipeline-logic: invoking binaries is what they're for. And it's very difficult to do that a declarative way: Make comes close, but it's difficult to track any side-effects that aren't "update these files", and a huge amount of CI/CD is no longer just "update a file".