Finding no collections when trying to sync shared icloud calendar

[martin-greentrax]

• Your vdirsyncer version
  0.19.2

• Your Python version
  3.11.8

• Your operating system
  Debian sid/trixie

• Your config file

[pair foo]
a = "foo_local"
b = "foo_remote"
collections = ["from b"]
conflict_resolution = "b wins"

[storage foo_local]
type = "filesystem"
path = "~/.local/share/vdirsyncer_foo/"
fileext = ".ics"

[storage foo_remote]
read_only = true
type = "http"
url = "https://p152-caldav.icloud.com/published/2/O....8gG-zj8sR....AzRG-5B...KeF-q0THP....h13c"

vdirsyncer --verbosity=DEBUG -c ~/dot/vdirsyncer discover foo 
Discovering collections for pair foo
foo_local:
foo_remote:
Saved for foo: collections = []

I'm trying to sync one-way a shared icloud calendar to my pc but I don't get vdirsyncer to discover any collections, nor do I know what I could put into collections. No part nor the whole of the uids of the url work. Neither does "from b".
When I simply hit the url I get a large ics file.

Is my config wrong?