My understanding (and personal opinion too) is that hybrid teams (half remote, half onsite) don't tend to work well. Most likely, the remote portion of the team will be left out of a lot small impromptu discussion that happen within the office. A solution is to force all conversation and discussions to happen via Slack/email but that makes a lot of small discussion not happen as the activation energy might be too high. Ultimately if you have a small portion of your team geographically removed from the rest of the team they might end up being left out a bit.

For example, let's say you have a team of 5 people, 4 people live in city A and 1 person lives in city B. It might be the case that the 4 people in city A meet up every week in person. They'll eventually form a closer working relationship with each other potentially leaving the person in city B out. Hashicorp's decision might be to try to prevent these kind of situations (for what it's worth I've heard about this Hashicorp rule by talking to people but I can't find anything online about it).