While that yields a correct-ish result, a couple of critiques:

• The spec didn't call for concatenation in the result set, so technically this didn't meet the requirement - but the test let you squeak by.
• Avoid expressing filtering conditions in JOIN predicate. JOIN predicate should involve key columns only. (Use WHERE to filter for full scores only. score is not a key in Submissions.)

Alternative solution, a bit more straightforward & easier to read:

SELECT
	h.hacker_id
,	h.name
FROM
	Hackers AS h
	JOIN Submissions AS s ON s.hacker_id = h.hacker_id
	JOIN Challenges AS c ON c.challenge_id = s.challenge_id
	JOIN Difficulty AS d ON d.difficulty_level = c.difficulty_level
WHERE
	s.score = d.score
GROUP BY
	h.hacker_id
,	h.name
HAVING
	COUNT(1) > 1
ORDER BY
	COUNT(1) DESC
,	h.hacker_id ASC
;