Also no way to block people that are not on your contact list. I get a lot of spam lately by scammers and the likes.

It's not and I'm not sure how that article arrived at that conclusion. Their E2EE crypto is problematic homebrew crypto, but that's very, very different from being closed. The whole desktop client including the implementation of that crypto is fully open source and lives right on GitHub. Plenty of people have independently reviewed it and came back with a very iffy impression of the whole thing.

Really the only difference is that Telegram doesn't publish their backend, but the one Signal publishes is missing a couple of bits related to their "spam filter", which happens to take in the source & destination of messages and do anything it wants with them. That doesn't matter for either platform's E2EE properties in any case, since distrusting the server is the whole point of E2EE.