Whilst I don't use Avast Secure Browser, I'm not a fan of Chrome or chromium browsers. However I would say I hope they don't learn this bad idea.
I'm not sure what you mean by modern devices, but I would guess you are talking about high resolution/dpi screens. Now it doesn't really matter how high your resolution/dpi is you still have to be able to read the content.
Example: my laptop is only 1080p on a 15.6" screen, if I were to leave it on the default windows dpi, I would have a hard time reading small text. So I have to increase the DPI in order to be able to read it and or change the text size in the browser. Both of these have the effect of reducing the screen width.
If the tabs were on the left, then this would further reduce available screen width, which has the knock on effect of making you scroll more to read all of the text.
However, with the tabs at the top, I have the full screen width, meaning less scrolling. So I hope you will see what I don't want this to become the so called default option without the ability to choose.