When Billy and Sarah walk into the conference room, the notebook in Sarah's arms switches directions, then switches direction a couple of times on the table.
In the flashback at the beginning it is explained, that Madeline wants Alfie to use the acronym MWBSP, instead of 'Mom would be so proud', so that she does not turn into a mess when she hears him say it. So the point is not to abbreviate the phrase. It's technically still not an 'abbreviation', though, as Madeline calls it, since it is one syllable longer, when spoken. They are both very smart and should know.