Susan Hill, Howards End is on the Landing
This seems a bit simplistic to me. I would argue that ideas tend to come in the form we have been trained, or train ourselves, to think them. Many novelists began as short story writers. While good short stories may be difficult to write, perhaps even more difficult than good novels, they are nevertheless an easier path for beginning writers to develop their narrative skills. Besides, few writing instructors want to read through student novels. Stories work much better for writing classes. Yet relatively few people actually read short stories anymore, and there are few markets for getting them published. Novels are where the money is, so writers who have trained writing short stories soon turn to writing novels. That first novel may be a struggle, but once they begin to think in terms of novels, it usually becomes easier. That is the form their ideas begin to take.
I noticed something like this in my newspaper career. As a beginning reporter I learned about what is called the inverted pyramid, putting the most essential information high in the story, then adding information of gradually decreasing importance. Soon I could attend a city council meeting and begin writing the story before the meeting was over, as well as any secondary stories that might have come out of the meeting. My ideas took the form of news stories.
Subsequently I became a book reviewer, a columnist and an editorial writer. Each required a somewhat different form of writing, and each required some discipline to think my ideas in that form. Now retired, I tend to think in the form of brief essays for this blog. This is not to say I don't occasionally get an idea more suited for an editorial or a newspaper column. This can be frustrating because those avenues are no longer open to me. Yet mostly my ideas tend to come in whatever form has become my focus. I question my ability to write a novel because, in part, I have never tried. My mind has never been trained to think novel-like ideas, no matter how many novels I have read over the years.
Once one has mastered a variety of writing forms and if one is not locked by circumstances into pursuing just one or another, then perhaps Hill is right. The ideas will come in the form they need to be written. For most of us, however, they will come in the form we know how to write. Novelists don't usually think like poets, or vice versa.