Cluttery is a word, but I've never heard it used in the U.S. We say "cluttered." I also don't agree that the second "t" in textbook is eliminated in pronunciation. We may say it quickly, but we do, in fact, say the second "t." I think she's Canadian. Canadian English is very similar to American English, but there are differences. Some of her advice about American words is not accurate.

yes it is used on odd occasions such as in your example it is using the suffix "y" in the same manner "y" is added to other words. https://www.yourdictionary.com/cluttery cluttery Adjective (comparative more cluttery, superlative most cluttery) cluttered Origin clutter +‎ -y so in your context it is just saying it would be very cluttered or cluttery

It's rare. I don't think I've ever heard it or seen it. Native speakers consult a dictionary for questions like this, which John has done. The built-in spelling check somewhere in my computer is putting a red line under it, so it's not in the built-in spelling dictionary, so it can't be common word. In English, it is not terribly unusual to use well-understood prefixes and suffixes to manufacture words, and "cluttery" would be understood by native speakers even if they had never seen it before. We can do our own instant research by performing a Google search. For example, if I search site:www.gutenberg.org I turn up a handful of examples. However, none of them are from authors I recognize. One of them is: "She said the room was too full, and looked cluttery; and she said that only country folks kept family pictures in their parlors." Google Books turns up more: "His kitchen is so bachelor-cluttery, it looks as if it hasn't been cleaned in years." "He had been in a cluttery old shed with someone he couldn't quite see." That is from a book by Stephen King, a famous modern writer of horror novels. '"He doesn't have those cluttery little beds of seedlings and slips about, either," she said. "His garden is always neat and trim."' English dictionaries are "compiled on historical principles;" they record the language as it is. On the evidence, I would say "cluttery" is a real word.