Scotland won their Nations League group whereas Bulgaria finished behind Norway and it's the highest placed non-qualifier from each group that goes into the play-offs (I think!). Norway currently only 4th in their Euro group.
So it's actually more complicated than that!
All the teams in each league of the Nations League were independently ranked based on the number of points they got in that competition. Bulgaria got the most points of any of the second place finishers in League C, so they were ranked 5th in League C.
For the Euro 2020 playoffs, they take the top four ranked teams from each league that have not otherwise qualified.
Scotland were ranked #1, so they'll definitely be in. Norway & Serbia (ranked #2 & #3) are probably not going to qualify automatically either, so they're likely in. But Finland (ranked #4)
are almost certainly going to qualify from Group J - they only need to beat Liechtenstein at home to nail it down. So the 4th spot will almost certainly go to the 5th ranked team instead, which is Bulgaria.
It actually gets even more complicated, because if any of the Leagues don't have four teams in them that haven't already qualified (and at the moment, it looks like only Iceland won't qualify automatically from League A), then they select more teams from the leagues below to make up the playoffs.
So even if Finland somehow screw it up, then it's
still likely that Bulgaria will make the playoffs as the 5th ranked team in League C because there won't be eight non-qualified teams in Leagues A & B combined.
Which all means that yes, Bulgaria will be in the Euro 2020 playoffs in March - so chucking them out would be a genuine punishment.
(although they looked so bad this week, that you'd bet on even Scotland beating them. I'd wager Serbia would absolutely demolish them)