That article says, "Failure to surrender, ie. not turning up on the date given on your bail sheet (whether to a court or to return to a police station) is a crime."

What's not a crime is breaching the conditions of your bail, e.g. you can't go to political protests if you're released on bail.

People interested in bail in England and Wales might be interested in the CPS page, which sets out who can get bail and why, and what counts as surrendering for bail or not: https://www.cps.gov.uk/legal-guidance/bail

In particular, https://www.cps.gov.uk/legal-guidance/bail#a19

This shows that breaching your bail conditions means you may be arrested and either re-bailed, or taken into custody.

That article is about police bail. Assange was bailed by the courts. The breach of court bail is an offence punishable by up to 12 months imprisonment.