This is true in a "technically correct" way, but not in a meaningful way.
It's like developing sophisticated websites for IE6-era web, with ActiveX and Java applets and this new "ajax" thing on the horizon that sure sounds nice but it'll be a decade before you can actually use it for most of your users.
The very core basics are essentially the same because yea - it's just a web browser. An will be bigger than a
. But they are regularly multiple years out of date, have WILDLY different security and native-access models, version specific bugs, initialization and threading requirements, performance tradeoffs, styling quirks, and you might have hundreds or thousands of versions to test against which you cannot reasonably test against because they are frequently tied to specific operating system versions that you can no longer download and install, or require hardware you do not have.
So yea. IE6-era stuff. Not an exaggeration at all.
For simple stuff they work just fine, performance is generally more than good enough, and they start up faster, use fewer resources, and lead to a much smaller install. They're entirely reasonable choices. But once you push the edges of the envelope they're an absolute nightmare, and that is the entire reason Electron exists. That is what caused Electron to become the giga-powerhouse that it is now. It solved that problem, at relatively high cost, but it is incredibly obviously worth it to anyone who has dealt with native webviews in complicated ways.
Modern browsers are a completely different game, in comparison - far more consistent and up to date in aggregate. They're utterly incomparable.