The Chief Rabbi is an extremely articulate and intelligent man, who would have given great thought to the consequences of his actions in speaking up against a proposed “leader” of the UK. Anyone not overtly mendacious and living in a bubble of denial precipitated by gross antisemitism, acknowledges that there is indisputable evidence to support the idea that Jeremy Corbyn has acted wholly negligently in his refusal to stamp out the rampant antisemitsm in his party. There is also evidence to suggest that Jeremy Corbyn has involved himself in antisemitism himself.
What you understand is irrelevant, because the examples set out in the IHRA definition make up the definition. Corbyn and the National Executive Committee of the Labour Party’s refusal to accept the IHRA definition shows that they have little respect for the Jewish community and their views.
A video has recently emerged of Corbyn speaking to none other than Iranian state-TV, essentially questioning Israel’s right to exist. Search for “Corbyn questions Israel’s right to exist”.
Your assertion that Rabbi Mirvis represents only a small segment of British Jewry is totally incorrect. As the spiritual leader for British Jewry, he represents the entire British Jewish community and Jewish communities all over the UK recognise that fact. The evidence to suggest that Jeremy Corbyn hold antisemitic views is compelling and the Chief Rabbi has every right, and is indeed obliged in his position as spiritual leader of British Jewry, to warn of the dangers of electing an antisemite into government.