Under IFAB rules, a challenge can still be a penalty if the defending player gets the ball first and the officials deem the tackle to be careless, reckless or has excess force.
https://uk.news.yahoo.com/celtic-fire-off-sfa-missive-175806472.html?guccounter=1&guce_referrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8&guce_referrer_sig=AQAAAILHqBWOTVb3CeFWFN94sh2w_cU05ljij_M6wQqJNglHOGYokswORxUMcwUfHMD9x2byWRHvAFz44tU7r1xi2hWvPFnma1hRIyrFk4v0WqNOVSCE3QfPfgXIcaYJ5kWNiYN97ey_NeSYjIj7kG4XhxkZojQyUSvLiBaKBy87thSd
Celtic obviously seeking clarity, for once I understand why they are asking for clarity. Usually i would put it down to appeasing their fan base but i think clarity is required.
I don't think the tackle was careless, reckless and there definitely wasn't excess force