Short Answer: You can consider the adventures ordered mostly in terms of difficulty as well as character level, but not all of them are "the deadliest in history".
Tales from the Yawning Portal is an excellent book; a collection of some of the great adventures in the history of Dungeons and Dragons. You're looking at a source for seven adventures, all varying in difficulty and length. The promotional description may be a little generalized though. A better description is A collection of exciting and challenging adventures that gives DMs resources for one-off adventures for all player levels. Sure, Tomb of Horrors is only for DMs looking to eliminate player characters, but the others run differently, and each is challenging in its own way.
The Sunless Citadel and The Forge of Fury are the lowest levels, and (relatively) easiest adventures of the book. Citadel is a great one off specifically for 1st level characters. The Forge is perfect as an immediate run right after The Sunless Citadel. These are perfect for teaching new players or those who have difficulty with traps how to handle themselves properly while dungeon crawling. Deadly rating: Not so much. Average difficulty.
The Hidden Shrine of Tamoachan is the next adventure in character level, but definitely takes a step up in difficulty. Traps are devious. The treasure inside is a little sparse, and may not be worth the danger. Even if the characters survive the adventure, there is a chance that the dungeon can trap them inside, and force them to be new slaves for Tamoachan! Deadly rating: Deadly. Beware.
White Plume Mountain mellows out the challenge a bit, giving an expected challenge for its level range. If the party keeps a level head, the incredibly creative puzzles and environmental encounters are not too overwhelming. They are definitely creative, and usually do not take a linear approach like standard traps. So if your players are not used to handling that, it will be difficult! Deadly rating: kinda deadly.
Dead in Thay is really a series of adventures in a super-dungeon, and expect this one to compete with Against The Giants for the longest. This one is a collection of very creative "rooms" that all roll up into a marathon of encounters that will drain your players of their spells and daily powers. Resource Management will be the silent killer in this one. Pay attention to the risk of character death in this place, because if certain conditions are not met, any resurrected character will find their soul trapped forever! Deadly rating: Deadly
Against the Giants is another one that really is multiple adventures. It is three different modules that will send the players on a tour of the different types of Giants. All of the Giants are dangerous in their own way, so if the players do not adapt their tactics and evaluate their enemies, then they could find themselves in trouble against one type or another. The difficulty evens out for this one though, as it offers some separation between modules, which means a chance to rest and recharge - something not readily offered in Dead in Thay Deadly rating: Not so much, Average.
Tomb of Horrors ... Evil. Just ruthless and pure hate. Run this if you promised your players that you would kill some of them. If your players have any trouble with handling traps or figuring out puzzles, it's all but guaranteed to be deadly. There are false entrances that may even be trapped which prevent the party from even entering! Inside, there's a seemingly innocent trap that levitates a player... into a chamber where their only real options are to starve or die of thirst, or run into another room to be assuredly chopped to bits. Then, there is another trap with a chance of being magically stripped of all of your possessions, and then teleported out of the dungeon entirely. Those are just two of the thirty-some encounters of this dungeon. 'Nuff said. Deadly rating: Over 9000. Side effects include: possible rash, intense weeping, may ruin friendships.