Etymology is a tricky business. Here’s a simple proof for doubting students:
English cognates are routinely formed from the present and participial stems of Latin verbs, often from both stems of the same verb. The present and participial stems of dēfundō, dēfundere, dēfūdī, dēfūsum are (obviously) ‘defund-’ and ‘defus-’. There are English verbs ‘defund’ and ‘defuse’. But neither is a cognate.