I would normally say that the week days after Memorial Day are usually not to bad yet because many schools are still in session. The actual weekend can be busy because it is a holiday weekend (most folks get Monday off). TP probably assumes that the crowds will be lighter on 5/31 and 6/1 because the SoCal Select AP blockout is in effect, but I tend to also think that means that
Disneyland assumes non-AP guests will fill up the parks.
Now, as
sherilaine pointed out, Star Wars Land is slated to open in the summer (so basically any time starting with Memorial Day weekend), with most guessing that it'll open in mid-June. If that's the case, I would guess that they'll be doing media previews, hard-ticket previews, etc. for a bit before the opening. If it doesn't open until mid-June, then the week after Memorial Day will probably still be okay, but if opens on Memorial Day weekend, then the crowds will be huge regardless of the day.
As others have said, TP is not that accurate for DLR. I think one of their biggest problems is that they use attraction wait times as their main metric. However, there is a sizable number of guests that go to DLR for the shows (e.g., parades, fireworks, F!, etc.) and other non-attraction activities. When they used to post assess their estimates, they tended to underestimate the crowds.