I mean, I sort of get why the developers say it's Discord's policy even if it's a bit misleading.
Game developers don't really want to moderate their own discord server and simply want to use the strictest automated filtering system available and this just happens to include phone number linking. The operators of the servers themselves do not have access to these phone numbers and they are only stored by discord directly to prevent spam.
I would personally prefer games to not have their communities tied to discord, akin to how forums were big deal for games back in the day, but even then they do need some kind of automated way to filter out all the crap. This is a problem with moderating any community, including a lemmy/kbin/mastodon, and I don't blame them for simply picking the strictest option to ease the burden on the 1 or 2 people who are charged with managing these servers (especially if they are unpaid or volunteers, which is a whole other can of worms that shouldn't happen...)