All events are time-boxed?

This is rather nuanced and the definition of timeboxing.

All Scrum events, including the Sprint, are timeboxed. That is, they all have a maximum length that cannot be exceeded. The maximum timebox for the Sprint is 1 month. Teams may reduce the timebox of the Sprint and scale down the timebox of the other events based on Sprint length.

However, most events may conclude when their objectives have been complete. The Sprint is also fixed-length in that the team does not end the Sprint when, for example, the Sprint Goal has been achieved. This maintains the scheduling and cadence of the Sprint Planning, Sprint Review, and Sprint Retrospective events that bookend the Sprint.

My understanding is that Sprint is fixed length and other 4 are time-boxed.

The Scrum Glossary says: "Sprint: Scrum Event that is time-boxed to one month or less..."

I was referring to Scrum guide

The Sprint

They are fixed length events of one month or less to create consistency.

To your point @Nitin, I don't think time box is used in the Sprint section of the Scrum Guide. It just says fixed length, but timebox is implied, similar to Daily Scrum (which doesn't specifically say timebox either).

The Time constraints section of the Scrum competency materials you initially referenced provides some supporting information: https://www.scrum.org/learning-series/scrum-events

Time constraints

To help create discipline and focus, each of the Scrum events has a predefined time constraint, or timebox:

• The Sprint’s timebox is no greater than a month, though they are typically two weeks in duration.
• The Sprint Planning, Sprint Review and Sprint Retrospective are timeboxed to 8 hours, 4 hours and 3 hours, respectively, for a one month Sprint. The team generally sets the maximum duration of these events to be less when the Sprint is shorter than a month.
• The Daily Scrum is 15 minutes regardless of the length of the Sprint.

As @Thomas mentioned, the Sprint is both timeboxed (less than a month) and fixed length in that you don't end the Sprint when the objective (Sprint Goal) is achieved like you do with the other events... 

If the team is able to achieve the purpose of the time-boxed events (Sprint Planning, Daily Scrum, Sprint Review and Sprint Retrospective) before the maximum time allotted is reached, they should simply end the meeting.

Thank you Ryan, your explanation is very helpful.