Not sure about your conclusions comparing Ryan and Dowd.
Comparing Capitals and Oilers zone time differences is significant. Capitals have 44% defensive zone time while Edmonton is at 38.4%:
NHL EDGE Puck and Player Tracking Statistics - Comparisons.
Dowd's offensive zone time is largely irrelevant as he is a defensive specialist which is reflected in his zone time. Hockey Reference shows him at 89.8% defensive zone starts which is massive. His shooting % last year was 21% with 13 goals and year before, 11% on 11 goals.
Another conclusion might be Dowd is an elite defensive specialist who is trusted for hard ice time and match ups but is also able to keep his head above water offensively. Offensive zone time measure alone can distort against players whose role and deployment is goal suppression.
On a team with -33 goal share, Dowd is +8 despite his zone starts and deployment.