The mistake there would be getting that specific about it – it's not a useful method for an individual developer, but it _is_ a useful method for the team overall, where these effects get amortised.

(Although in this specific case, your colleague who has been working on one ticket for two or three weeks is operating in a way that I find is usually pretty harmful for productivity overall. "Solving a critical bug" is almost universally something that can be broken down further.)