Thanks!
The original tutorial has direct controls that add a fixed amount to x/y. My changes introduce acceleration and drag, which does make it slippery. In fact, max speed is only a soft cap because of the proportional drag.
This could be changed to only apply drag when not moving and go back to direct moving. I would do that by just setting velocity with the arrow keys, which would give a snappier start to the movement and quicker direction changes.
However, if the drifting stop also contributes to your feeling of slippery controls, then the whole thing would need to be binned and go back to only direct movement. The ultimate reason for the change was to allow faster movement without requiring twitch gameplay. Watching people play it and it was too hard with the direct movement style, hence the change.