That is entirely false. Replacing the mainboard itself costs the same amount of money as a new laptop (an entire device). Their component prices are on their website under "Shop Parts", so you can verify that for yourself. I can buy a brand new Ryzen 7000 series laptop for the price of replacing a Ryzen 7000 series mainboard for a Framework laptop. Their laptops are also a lot more expensive than same spec branded ones from Asus, Lenovo and Dell that have better build quality and design.
I don't know where does this myth come from. The cost of replacing individual component is more expensive than replacing an entire device which people do not do because it needs repairing or often even upgrading, but because they're sick of the sight of it. You can't replace one component and extend the life of your PC another full cycle because you'll soon have to replace other components too. So when it comes to upgrading you have to consider the price of upgrading all available components to get the true cost as opposed to buying a new device.
Eventually, sooner rather than later, both RAM and SSD will come soldered on, so the only thing you will be able to replace is the battery and the screen. Both which 99% users never have to replace.
I am a Thinkpad user myself, have had them for both work and pleasure. Recently upgraded my old T14 for an X13 after reading and watching a lot of Framework reviews. It's just simply a gimmick, with a lot of quality issues, being sustained by having LTT name behind it.