The assertion only 1% are valid claims is patently absurd. You can look up criteria for an acceptance of claim on the gov.uk website and caseworker guidance and civil servants do boringly follow rules and guidance.
Also a fair amount of accepted claims are deemed so by courts at appeal - sometimes the Home Office withdraws rejection as it feels the decision was wrong and then reconsiders, sometimes they’re told to do so, sometimes the appeal court changes the decision straight off.
It will be debunked but no one will notice that will they.
98% staying after claim is rejected is unclear - have they an appeal that they are entitled to stay for? Or just saying those at end of appeals process stay - that would be a massively smaller number than that implied - although yes, returns need to be dealt with appropriately and if you check stats these have increased considerably under Labour.