The sloka mentioned by the OP is a popular Guru Stotra sloka from Viswasara Tantram.
Brahma, Vishnu, Shiva are considered to represent 3 Gunas, ie., Rajas, Sattva and Tamas. And, Brahman is termed as trigunAtIta,ie., beyond all Gunas.
Guru is the one, who attained Brahman, and thus BRAHMAN itself. So he was eulogised as the equal to Brahma, Vishnu, Shiva....
Is there any Vedic verse that explicitly says Brahma and Shiva did not exist before creation?
No. As per the Vedas and Upanishad. Shiv, Vishnu and Brahma are one. For ex:-eighth mantra of Kaivalya Upanishad.
स ब्रह्मा स शिवः सेन्द्रः सोऽक्षरः परमः स्वराट् । स एव विष्णुः स प्राणः स कालोऽग्निः स चन्द्रमाः॥८॥
He is Brahma, he is Siva, he is Indra. ...