A DSLR is not strictly necessary for digiscoping. People do it with compacts, bridge cameras, and even phones; the key tradeoff is image quality versus cost, size, and convenience.

From the comparisons mentioned, a dedicated telephoto lens still wins if you want the best possible image quality. Digiscoping is more of a compromise: typically lower image quality, but often much cheaper, lighter, smaller, and quite weather resistant.

A bridge or compact camera can be “good enough” for digiscoping, especially if your priority is reach and portability rather than maximum image quality. A DSLR may improve results, but whether it is worth it depends on what limitation you’re trying to fix now: resolution, noise, low-light performance, or something else.

Also note that common afocal digiscoping setups can be awkward to use, regardless of camera type.

So the practical answer is: no, a DSLR is not required. Choose based on your priorities. If absolute image quality matters most, use a telephoto lens. If cost, weight, and long reach matter most, a spotting scope with a bridge/compact camera can make sense. Testing with an adapter and an inexpensive older DSLR is a reasonable way to compare before spending more.