When this happens to a rock a glacier picks up, it is called a "dropstone".
During glacial maxima, icebergs can survive much farther south than now. The presence of dropstones in what were supposedly equatorial sediments led to the theory of a Snowball Earth a billion or so years ago.
Based upon CO2 data, 100,000 years ago, the Earth was working its way into the most recent glacial maximum, called the Wisconsin glaciation in North America ( the likely source of this iceberg).
Source: Wikimedia Commons, Author Tom Ruen, CC BY-SA 3.0
Notice that the article did not call the tusk a "fossil". You can only call it that after a specific process of mineralization after being buried in sediment. They might very well have found original ivory from the mammoth.
But this is not necessary. There are two ways the tusk got into the glacier:
The carcass was buried in sediment and some or all of its skeleton, including the tusk, became fossilized. Later, the glacier picked up the rock containing the fossil.
The mammoth died on top of the glacier, the carcass was buried in snow and more glacier may have built itself on top. This is the most likely scenario if they found original ivory.