Seems a bit of a mish-mash. "Subtle" is definitely of latin origin, and most romance languages still pronounce the b - per the article, "debt" is actually in the same ballpark. "Island" however is of proto-germanic origins (ieg, iegland) and the coincidence with latin "insula" which later gave French "isle" and from there English "isle" is accidental.
[0] https://www.etymonline.com/word/island
Edit: note about debt though - in most Romance langauges today, there is no b: French dette, Spanish deuda, Romanian datorie, Catalan deute; Portuguese and Italian do seem to use debito, with the b, though. The situation is similar with subtle: we have "subtil" in French, Romanian and Catalan; but sutilo in Spanish and Portuguese, sottile in Italian.
We should also note that Romance langauges tend to prefer phonetic spellings to etymological ones, with French being something of an outlier. Even there, the extra letters that are written are typically still relevant in declinations/conjugations, such as in mauvais (masculine, silent s), mauvaise (feminine, se pronounced like z).