I'm curious if anyone knows how this works?

If Boston looses to Washington they will be the 4 seed in the East, but they had the best record

so if for some reason they don't make the conference final there pick would have been 27th OA
(the two losers in the conference final would get 28 & 29, the 2 Stanley cup teams would be 30 &31)

But since they are actually loosing the round Robin will that have any effect on their draft spot?

Short answer is no.

Long answer is:

These are the rules for how the draft is ordered:

1. The teams that did not qualify for the playoffs the previous season (picks 1–15)
2. The teams that made the playoffs in the previous season but did not win either their division in the regular season or play in the Conference Finals (picks 16–23 up to 27)
3. The teams that won their divisions in the previous season but did not play in the Conference Finals (potentially picks 24–27)
4. The teams that lose in Conference Finals (picks 28 and 29)
5. The team that was the runner-up in the Stanley Cup Finals (pick 30)
6. The team that won the Stanley Cup in the previous season (pick 31)

2, 3 and 4 are ordered by points from worst to best.

Since Boston won their division, they will pick either 27th, 29th, 30th or 31st.