This is not HSX's official criteria for How/Why starbonds are attached (or not attached) to moviestocks, but my own observations and thoughts.
The purpose of starbonds is to see how much "star power" a performer/director brings to a project. Does a movie make more or less box office just based on who directs it and/or who stars in it?
Starbonds are credited to moviestocks and adjust when their "star power" is used by the movie to advertise it and sell it. This is done when they are listed on the movie website; listed in the movie starting credits or end credits before the roll; listed on the movie poster; listed in trades movie reviews as cast; listed in movie production notes/press materials; casting is announced in trades ...
Cameos are small performances by a known performer that are not advertised and meant to be a suprise to the movie audience ("Hey, I didn't know she was going to be in this movie"). Cameos are usually not adverised so that/those performer(s) do not use their star power to market the movie and are not attached to the moviestock and do not adjust.
John Slattery is not listed on the movie website for Captain America:Civil War. Not on the poster. Not in the before the credit role credits. Not in the production notes. His casting was not announced in the trades. Nobody went to the theater to see Captain America: Civil War just because John Slattery was in the movie. His appearance in Captain America: Civil War was meant to be a suprise to the audience while watching the movie.
" In addition to cameos by John Slattery and Hope Davis, the segment features ..." (link)
John Slattery was listed in Marvel's press releases for Ant-Man. His appearing in the movie was announced and was noted in the trades:
"First Look At Paul Rudd In Marvel's Ant-Man" (Variety) (link)
Evangeline Lilly, Corey Stoll, Bobby Cannavale, Michael Peña, Abby Ryder Fortson, Judy Greer, David Dastmalchian, Wood Harris, John Slattery, T.I., and Gregg Turkington also star.
Judy Greer, John Slattery, Wood Harris, Rapper T.I. Join Marvel’s ‘Ant-Man’
(The Wrap) (Link)
It was important for people to know that John Slattery was in Ant-Man because that is how Ant-Man is linked to the Marvel Cinematic Universe. Agent Carter was considered a cameo role (a suprise). HSX was not sure about attaching Anthony Mackie... he started appearing in the trailers (to sell the movie) but HSX still waited to see if he was credited in the credits before the end roll (link).
Marvel did not want you to know that Slattery was in Captain America: Civil War because they wanted it to be a suprise... and starbonds do not adjust for that.