I disagree with this as a blanket statement.  Yes, sometimes it is true, but often, it's not.  It depends on where the vote is placed.  You need to know more about the post/comment to know whether it was spam or not.

For example:

My 15 year old son, @cmp2020, has studied piano for the last 6 or 7 years, and music theory for the last year.  When he posts on those topics, his self-vote is far more valuable than many other votes in conveying information to the system.  Similarly, at a higher order, @lemouth is a particle physicist and @justtryme90 is a biologist.  If they want to promote their own comments to the top of a discussion in their area of expertise,  they should certainly be able to.

I'm sure that almost everyone  on steemit has specialized knowledge on some topic or other, where their own self-vote should carry more weight than the vote of a random person who happens to see the post/comment.