I don't really think anyone is copying anyone here. This is a popular feature for browsers that has been around for over a decade.
Orion[1] is another recent MacOS browser that released before Arc in 2021 and has vertical tabs. Edge had vertical tabs as an experiment back in 2020[2] and is full feature now. Vivaldi has had vertical tabs since 2015[3]. It might not count since it's not built in, but Firefox has had vertical tab extensions since 2007[4].