In theory, this might be true. In practice, I find that it is just an excuse to cut corners and ignore the problems.
I recently had to help somebody with their public library and a particularly magazine publisher for digital issues. They require a web app to read the issues. They were trying to use their Mac, but the site refused to load. The public library staff had no clue why it didn't work ("well, it works for us", and their attempts to contact the publisher was your stereotypical tech support horror story.) Long story short, their website doesn't work on all browsers. They implicitly know that it doesn't work in Safari because they apparently wrote a native app just for iOS. However, they have no native app for Mac and never bothered to fix their website.
I recently had to help somebody with their public library and a particularly magazine publisher for digital issues. They require a web app to read the issues. They were trying to use their Mac, but the site refused to load. The public library staff had no clue why it didn't work ("well, it works for us", and their attempts to contact the publisher was your stereotypical tech support horror story.) Long story short, their website doesn't work on all browsers. They implicitly know that it doesn't work in Safari because they apparently wrote a native app just for iOS. However, they have no native app for Mac and never bothered to fix their website.