You are probably familiar with Parkman’s “Oregon Trail”, which has a similar account of plains grizzlies encounters in the late 1840s.
I live adjacent to prime griz habitat(Purcell Conservancy) and see grizzlies and their cubs most summers. They are magnificent creatures.
And fast. Faster than you would think.
By all accounts, they are scary fast. I chuckle when I see a man being chased by a bear in a movie...I always think “No Way.”