To go unbeaten in a league like England, which is without doubt the most competitive in the world, is a fantastic achievement.
To put it into some perspective, the great Barcaelona side under Guardiola couldn't do it in La Liga, which is much weaker, and where they only have to play Athletic and Real Madird twice.
Though, a side should only really be called the 'Invincibles' if it goes unbeaten in every competition. That would truly be a remarkable achievement.
Just look around Europe, going the season unbeaten domestically isn't that uncommon, particularly in poorer leagues. As the Scottish league gets poorer, I expect there to be more unbeaten seasons in the future.