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Yes. But with pressure and chemicals they can pretty well be cleaned.

In much the same way they (or in some cases don't) clean the lines at your local bar.

those amateurs. they should pour the beer into bowling balls and then roll those along the pipes.

there's an industry that deals with this problem:

https://en.wikipedia.org/wiki/Plumbing

Here's what they'll actually use for that: https://en.wikipedia.org/wiki/Pigging

Poly pipe is pretty good for being non-porous and difficult for much to grab onto or grow inside - Oil & Gas uses poly lined steel pipes frequently where microbially induced corrosion is an issue (various extremophiles that live in the reservoirs that like to eat iron and shit acid, etc).

clean-in-place doesn't usually use pigging, just a nice spray of caustic and peracetic acid does the trick.

At that length I'd be surprised if they didn't - caustic & acid will require a few line flushes etc, whereas a plastic cleaning pig can be run with the beer in line without quality issues provided they run it every few days.