Fair question. I think that the Bangladesh team we've seen in this test is unlike anything they've managed before. The beginning of it was the win over Zimbabwe I think, and since then they've beaten India, Australia and Sri Lanka in ODIs, and pushed Sri Lanka in a test series, and now this.
The point about the "tough at home" comment was specifically about non-subcontinental teams. Traditionally, teams like Australia, England etc have a tough time touring the subcontinent, because of the unfamiliar conditions, environment, food, crowd and so on. Bangladesh have never been considered part of this, it's just been India, Pakistan and Sri Lanka. However, I think one of the things we'll see in the wash-up of this series is that the "Bangladesh tour" will be considered a difficult venture in the near future. It will be a long time before Bangaldesh are going to go to England and win a test series, but like Sri Lanka in the early 90s I think they'll be fairly unthreatening on the road, but a real danger side at home. You can see from this match how well Bangladesh have adapted their play style to suit the home conditions, and how much it has benifited them.