Maybe we should call it a Secondary Public Offering then.
Pedantic nit: not quite an IPO (Initial Public Offering) in its true sense since the original ARM Holdings was already a public limited company (PLC) before Softbank bought it from its shareholders in 2016. The trading entity and group parent is Arm Ltd, a private limited company formed in 1990 [0].
There is however a currently dormant Arm Holdings Ltd [1] incorporated in 2018 which may be converted to a public company so it would be the first time this legal entity has been publicly listed.
[0] https://find-and-update.company-information.service.gov.uk/c...
[1] https://find-and-update.company-information.service.gov.uk/c...