I hadn't considered that your visa was issued in the UK - in that case they might indeed consider you as "transiting" through Italy (though if the visa was issued some months ago they are not to know whether you moved back to Italy in the meantime).

In any case if you want to take the Sichuan airlines (HEL-CTU) option then the simplest thing is to follow my advice above, and the advice in the message you received from the PCR test company, and allow a decent length stopover in Helsinki, and don't try to squeeze in the tests on a <24 hr transit.