Is the bonus room in Oil Drum Alley missable?

The sources I have found do not say anything specific about whether or not this is the case (aside from the page you provided in the question).

This speedrunning guide states that this room is required to complete 101%, but it is not necessary to complete it; simply to gain access to it.

Additionally, the guide also states:

When the barrel drops from lining up the 3 bananas, DO NOT run at the wall. This will result in the victory animation and you won't actually make it into the bonus. You need to jump into the wall instead.

This implies that entrance to this level is bugged, and requires specific manoeuvring to gain access to the room.

I also investigated any possible changes between versions of the game, and this appears to be the same, regardless of version. (No mention of this level being "fixed" in alternate versions of the game).

Edit: after playing this, I have determined that this is in fact not "missable". I tested this by:

• entering the "sub" bonus room, and failing, dying, then returning
• failing the main bonus (so I could not enter the sub room), dying, and returning
• entering the "sub" bonus room, completing it, dying and returning
• entering, failing, and completing the whole level
• entering, completing the bonus, and completing the level.

In all of these instances, I was able to return to this room. This makes it an excellent way to farm lives.