Not exactly. As I understand "he has gone off on a tangent", it means one who changes the subject, (perhaps unintentionally due to habit), I think:
"The students complained about the professor, because he always goes off on a tangent and starts talking about unrelated things"
يخوط بصف الاستكان means one who doesn't actually understand or misheard the main subject, so they discuss a different irrelevant subject they think is related, sometimes intentionally (by acting they haven't understood or heard): (I never use it, so I will quote what I once heard)
المتظاهرين يريدون إصلاحات وتعيينات وكهرباء وانتخابات مبكره وريس الوزراء يخوط بصف الاستكان يريد يطلع الامريكان
The (Iraqi) protesters demand repairs, jobs, electricity and early elections, but the prime minister is stirring far from the cup as he tries to take out American (troops from Iraq).