It is, but I don't see how web browsers would need to interact directly with ActivityPub. That's just a way to settle on a json structure everyone will be using in their web application (that acts as AP client), as is the case in Mastodon.
Here with PeerTube the client interface doesn't interact with AP to watch videos or get them. It just requests the list directly to the server.
ActivityPub defines federation messages server to server and client to server. It is not a protocol per se, and rather a message exchange standard, which could perfectly be used only between servers, as is the case with federation of videos between PeerTube instances, and more recently for video comment feeds, that can interact with the larger fediverse (Hubzilla and Mastodon so far were tested).
In no way it defines how you access media. That is defined by the use of WebRTC, which is supported by a growing number of browsers, and anyway provides a fallback to direct streaming (HTTP), so that any browser can interact.