I think i would rate this a bit higher at maybe 7 for two reasons
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A 600m higher credit line is quite significant for Sixt and its good that they can extend it at favorable conditions (good banking relations and BBB rating)
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It might indicate more ambitions to grow and that things are going well. (More revenue growth and larger fleet in 2026)
More financing could in general also hint at problems in some circumstances but I think given Sixt predictable business model, the terms of the financing etc. this can be ruled out for this case.