Exercise 3: GRUB_TIMEOUT Edited Correctly, But No Change After Reboot — Possible Solution
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Most likely cause: they forgot to run sudo update-grub after making
the edit.

Explanation:

The chapter's own warn-box is explicit that this is a real, distinct
mistake from editing the wrong file entirely -- editing
GRUB_TIMEOUT in /etc/default/grub genuinely is the correct file and
the correct setting, so their instinct that "I edited the right file
this time" is accurate. But per the chapter's own core lesson,
nothing in /etc/default/grub actually takes effect on its own --
grub.cfg (the file GRUB actually reads at boot time) is only ever
updated when grub-mkconfig runs, which on Debian means running
update-grub deliberately after any manual edit to /etc/default/grub.
If that command was skipped, the OLD grub.cfg -- generated before the
edit, still containing the previous GRUB_TIMEOUT value -- is exactly
what GRUB reads and uses at the next boot, regardless of what
/etc/default/grub now says. The fix is genuinely correct and sitting
in the right place; it simply hasn't been applied yet, exactly the
distinction the chapter draws between this mistake and grub1-2's own
"edited grub.cfg directly" mistake.

WHY THIS WORKS AS AN ANSWER
------------------------------
This identifies the specific missing step (update-grub) and explains
mechanically why the old grub.cfg would still be in effect without
it, explicitly distinguishing this scenario from the different
mistake grub1-2 covered, rather than conflating the two.