Under opposed tests, it says "each participant makes his own separate test as normal, and whoever scores the greater number of degrees of success on his test wins the opposed test. Note that it is likely the tests are undertaken using different characteristics, and possibly with differing difficulties." (p. 14)

Under Demolitions (p. 95) it says "When explosives are placed, the GM notes the total degrees of success and the roll. This is used when diffusing those explosives, with an opposed test using the noted roll and degrees of success as the roll to oppose"

The latter seems to say that the bomb-defuser rolls against the bomb-creators die roll as their difficulty. For example, if the bomber rolled a 23 with 2 degrees of success, the defuser would have to roll under a 23 and get more than 2 degrees of success.

Shouldn't it be that the defuser rolls against their own target number, and has to beat the bomb maker's degree of difficulty?

You're misreading it. The defuser rolls their own Demolitions test (using their own target number), but must score more DoS than the bomber in order to pass. Effectively it's an Opposed Roll where the rolls take place at different times and the bomber's roll applies a penalty to the defuser, but that's a jumbled mess so they wrote it as they did.

Thanks!