Yes, they do need to be fitted. A treeless will fit a wider variety of horses, but they still need to fit. Different treeless saddles are suitable for different types of horses. Ex: Bob Marshalls are good for slightly wide to "normal" horses, but not great for super wide (think drafts or haffies) and not great for really shark withered TB types. Barefoot saddles have a changable pommel that will work for narrow horses to super wide, though not for really high withered horses.
Some treeless saddles have a weight distribution system, like Freeform, Sensation, Barefoot, Startrekk, etc. Some do not, like Tosrion, Bob Marshall, Black Forest, etc. They all need special saddle pads, though the ones without any panels or weight distribution systems need a more substantial pad. Treeless pads should have firm foam panels on either side of the spine, to create spinal clearance and weight distribution.
Any size person can ride a treeless. I weigh 250-270, depending on when you catch me, lol. I'm working on losing of course, but we all know how that goes...
My DH is 240, and he also rides my treeless. I use a Bob Marshall Endurance saddle with specialized Treeless Skito saddle pad
AND a ThinLine Western pad
underneath. My Skito has 3/4" firm foam with 3/4" lamination (creates more spinal clearance and better weight distribution for heavy riders). The ThinLine pad is used under or over my Skito, for added weight distribution and makes things non-slip. I also use a roper style mohair girth and an over-the-neck style breastcollar for trails. The set-up is very secure and my horses' backs are always comfy, even on long 4-5 hour trail rides.
Horses lacking in topline (spine sticking up and/or hollows behind the withers) can use a treeless saddle, but they need more creative padding while they build muscle. Treeless saddles can actually be very beneficial for these horses as their backs are protected by layers of foam from the rider's weight, and their muscles stay comfortable, which makes the horse more likely to use their back and hind end more effectively. Most treeless saddles conform to the horse's back, so extra shims are needed for "dippy" or swayed horses, so you don't sink too far and end up trapped by your saddle, lol.
If you want to try a treeless, I recommend finding a good dealer. They will work with you to find the right saddle for you and your horse, and you can demo the saddle and pad(s), to make sure it's going to work for you.