Slavery wouldn't have been slavery if the slaves could have legally moved to Canada. So, no, it wouldn't be slavery.

Slavery is a pet peeve of mine and I don't like the term being used for anything other than real actual slavery, which remains a real phenomonom in parts of the world and risks being underestimated if people start thinking of less severe cases as "slavery".

They can move to another country. They are free.

It would be oppressive, but it would be very different indeed from real slavery.

I wrote about slavery here:

http://citizenleauki.joeuser.com/article/344720/The_Arab_Empire_and_Slavery

In the article I mention Francis Bok, a former slave who fled Sudan and now lives in the US. If he hadn't been a slave and could have moved to the US legally, he would have been in the same situation as a 1%/100% candidate envisioned in the blog post.

I.e. if he hadn't been a slave, he would have been like one of the 1%/100% people.

The danger you describe is real and it would be oppressive and could still be sold as "fair" to the sort of people for whom "fairness" is the idea that other people work for free. But slavery it is not.

When one owes money to another he in fact, is a bond slave to that person or entity until he's free and clear of any debt owed. 

So maybe in the example offered here we could say that the 1% would be oppressed in this country much like slaves were slaves in the south until they got out from under their masters and escaped to the North, then they were no longer considered slaves. 

Right now we are in bondage to Russia and China especially because of the debt owed.  They are not exactly friendly towards us.  How does that make you feel? 

Who in their right mind  goes into debt bondage with an enemy or a potential enemy? 

Recently China has come out with a new program.  They are actively seeking to buy as much U.S. debt as they can by allocating great sums of money to do so taking advantage of our limping economy. 

So it really doesn't matter about the 1% because in all reality we are all enslaved by the massive debt owed to our enemies and it's only going to get worse.  We are so deep we are not getting out of this one.  If China and Russia decide to collect, knowing we can't pay what is that going to mean to the rest of us? 

No, not if those people had a choice about whether they worked. If they were forced to work at 100% tax, then there would be a case for slavery.

Edit: Your OP is slightly unclear to me though - are these people taxed at 100% of their marginal income above a threshold, or do they lose all their income? If they lose all their income, then how would it work, since they could just work less so they weren't part of the top 1% (in turn lowering the threshold for entering into the top1%, potentially down to 0)