Speed is not measured in stolen bases. Speed helps with stolen bases, but there is a lot more at play there than just speed. SBs are their own skillset.
And it does seem disingenuous to count Dave Roberts in 2004 but not Danny Santana in 2021.
Despite leading the team in steals, Vazquez was the second slowest runner on the 2021 Red Sox, with a speed of 25.2 ft/sec, barely finishing ahead of Travis Shaw (25.1 ft/sec) and being somehow behind Kevin "The Human Glacier" Plawecki.
The Sox did have some fast players, with the fastest being Jarren Duran by a lot (29.2 ft/sec), followed by Franchy Cordero, Jose Iglesias, Bobby Dalbec and Christian Arroyo to round out the top 5. Bogaeerts, Renfroe and Kike also all finished above league average, and Verdugo was almost right on the league average.
Source: https://baseballsavant.mlb.com/leaderboard/sprint_speed?year=2021&position=&team=BOS&min=10