It doesn't mean the same though, 2.6k implies somewhere between 2550 and 2650 years ago and 2600 an exact number of years.
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I would guess that sentence: "that sank 2600 years ago" would imply that we are not talking about exact date. But more explicit word like: "that sank about 2600 years ago" would help.
Using different number formatting instead of word or sign (~) is still implicit and not explicit.