https://en.wikipedia.org/wiki/Special:EmailUser/TheBigBoss3900 used to say "This user has not specified a valid email address." This is MediaWiki:Noemailtext. It was quoted 14 August 2018. At some time since then, it started just saying "Enter username of recipient" above a username field. If you enter TheBigBoss3900 in the field then the url changes to https://en.wikipedia.org/wiki/Special:EmailUser and the page says "This user has not specified a valid email address". Special:EmailUser/TheBigBoss3900 should also show this helpful message.
Similarly, https://en.wikipedia.org/wiki/Special:EmailUser/Asciident should say "This user has chosen not to receive email from other users" (MediaWiki:Nowikiemailtext). It used to say this, and still does if you enter the name in the field.

If the user can be mailed then Special:EmailUser/name gives the same result (a mail form) as entering name at https://en.wikipedia.org/wiki/Special:EmailUser, for example for https://en.wikipedia.org/wiki/Special:EmailUser/PrimeHunter.

It's OK that https://en.wikipedia.org/wiki/Special:EmailUser/TheBigBoss3900 gives an option to enter a username but it should be in addition to saying that the already specified user cannot be emailed. I don't know a MediaWiki feature which gives a link to Special:EmailUser/name when name cannot be mailed, but the link can be made in other ways like a template, user script, or manual url change.