They are different games... yes.
Open World aspects very similar... BotW introduced (atually revolucioned) in Open World some things not saw before and Elden Ring follow the same:
- Dicovery: the sense of explore and reward for that so you have fun exploring and that makes you want to do more... the gameplay loop become addictive and lead players to explore even more... that in ncentivizing player discovery was something added by BotW that Elden Ring used pretty well.
- BotW allows you to interact with the world... and it even push you to interact with the world... so the players start to swing their sword or try interactions with anything in the world... again introduced by BotW open world and so Elden Ring used to make it own world.
- Allow players to easy and fun to travel the worlds... in Open World before you traveled the world because you need and most players wanted warps zones and teleports to jump these boring parts... in BotW you want to travel because the system and options are great... the climbing system is one of the best saw in a open world game (most doesn't even have one), hourse ridding, shield surfing, paragliding, etc add even more to players wants to travel in that world... Elden Ring used the same ideia and made Torrent a double-jump horse that travel at extreme high speeds.
- Freedom: man that is probably the big change in Open World games introduced by BotW... freedom to do what you want whatever you want... the physics is so great that only your imagination is really a limitation to create unique experiences and creative solutions to the problemas you will face in the game... all shrine puzzes can be done in limitness ways.... you don't have the same freedom in Elden Ring as you have in BotW but they tried to avoid that weakness with a strong character build system that allow you to create a myriad of playstyles.
Both Open World design philosophies are similar... that is no surprise to anybody because Elden Ring took inpiration in BotW open world design.
The biggest difference in both games are not the Open World.
It is the combat system... or the value you get from the combat.... Elden Ring has a more complex and challenging combat system that can give you more satisfaction when you beat enemies... BotW choose a simple way combat system.
I can understand that most people that says BotW is not a good Open World game is probably not talking about Open World itself but how the combat gameply didn't fullfill their needs... something that Elden Ring does way better.
Open World itself BotW and Elden Ring follow the same designs and are very similar in execution.