Magnitude of instantaneous velocity = $|dr/dt|$ but this is not always equal to $ds/dt$ where ds is the infinitesimally small change in distance in the interval $dt$
Which one is speed?
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For both you are only travelling an infinitesimal amount.
The difference between $\Delta r$ and $\Delta s$ goes away as $\Delta$ becomes smaller. In this case it's $ds$ and $dr$ which is the limit where they become equal.
You can look at your drawing and imagine as you decrease r and s, the space between the line $dr$ and the arc of the circle will keep getting smaller until at an infinitesimally small value they are the same.