[samtheman]

but has since come onside
—
That’s a weird phrase.

As in not offside.

Does it mean “on the same side”?

Was the writer conserving alphabet?

[RummyChick]

It is not weird. It is a common turn of phrase. Perhaps one of those Brit/American differences.

[SamuraiScot]

but has since come onside — That’s a weird phrase.

The Guardian is Brit. An obsessively socialist twit-fest, yet when writing about things that aren't political, can be quite intelligent and entertaining. (Nothing like the Gray Lady of New York.)