These cells were exposed to insulin "chronically" (repeatedly, regularly, constantly, etc.) in vitro (in a cell culture) and became insulin resistant. It's not one dose of insulin. It could have been a steady flow of insulin, or repeated large doses over a period of days.

Steroids like dexamethasone are known to raise blood sugar, and the human body always has a baseline level of insulin circulating within it (because insulin is the key that allows glucose to enter the cells and supply them with energy). This is your "baseline metabolism." So perhaps the authors were trying to simulate exposure to steroids in the presence of insulin (i.e., matching the typical in vivo picture) and explore the mechanism for elevated blood sugars in steroid-treated patients? The sentence indicates that insulin resistance resulted, which would be consistent with the typical clinical scenario and would indeed cause blood sugar to rise (as without responding to insulin, glucose can't enter the cells and remains in the bloodstream).

So here, they are simply applying insulin to the cells in the culture, and other sections may clarifying the relevant dosing schedule and dose amount(s).

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Note added at 9 days (2021-11-23 05:49:09 GMT)
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may *clarify