The example druid has a total of 2 daily spells across any terrain(s) they visit, not 2 per terrain.
No, the example druid does not get to cast 2 daily spells from one terrain, move to another terrain, and cast 2 more. They can cast 2 cumulatively for the day, but dictated by the terrain(s) they visit.
The omitted part of the paragraph you quoted explains how this works (emphasis mine).
As indicated in the Terrain Caster Progression chart, at any given level you can also cast a certain number of daily spells. You don’t have to choose them or memorize them like wizard spells, cleric spells, or the spells from other druid talents—instead, you can cast any daily spell of your level or lower that matches the terrain you are in.
The daily spell quantities indicated in the progression tables are daily limits to the total number of spells the druid can cast, but the particular spells they use are limited by the terrain(s) they are in. When the number of daily spells they can cast exceeds the number of options they have available, they simply end up casting one of the spells more than once, but in general they could choose to spend all of their daily spell uses on any combination of available spells, even choosing to repeat the same spell multiple times.
So, for the example of the 2nd level adept terrain caster druid that you described, they have a pool of 2 daily spell uses, but at the time of using a daily spell from that pool they must pick which spell is cast, choosing from the spells of their level or lower for the terrain they are currently in (or the feat spell for that terrain if they have it).
Therefore, a low-level druid with only 1 or 2 daily spells may not be able to display much variety in their choices unless they happen to visit a lot of different terrain types. Otherwise, if they are always in the same terrain, then their selection would be limited to only the handful of spells available in that terrain.
As for this sentence...
Consequently, low-level druids may prefer to travel or operate in areas that include many different types of terrain.
There are two factors that directly influence the spell variety available to a terrain caster in a day: their level (hence how many times they can cast) and the number of terrains they visit (hence which spells they can cast). These compound each other, so a low-level druid that stays in one place happens to have the least spell variety in a day. That's what the sentence is conveying.