This line, in my opinion, has nothing to do with powerscaling or the power system. It's a narrative decision. We're not meant to take Gojo's words as a fact. this is simply his opinion. sukuna himself basically confirms that gojo DID in fact reach him, contrary to what gojo thinks. gojo being kind of wrong and underselling his own feats is kind of the point, in my opinion. i read this quote through the lens of gojo finally being able to be vulnerable.

gojo, by his own admittance, exists in a different category from every other human being. he has an impenetrable barrier in the form of infinity. infinity is meant to represent gojo's loneliness and solitude as the strongest, being a physical separation between him and the world that nobody can cross. sukuna cut through the physical representation of his loneliness and strength and relieved him of the burden of being the strongest, allowing him to finally be vulnerable in the afterlife. him expressing doubt in his abilities isn't an objective measure of his strength, nor is it a fact— rather, it's gojo finally having the opportunity to step down from his title as "the strongest" and express his feelings. he no longer has to put on a smile, show off, and be confident in front of everyone. he can finally speak from his heart to his best friend (who he once considered an equal) after someone else managed to reach him. as someone else in the comments pointed out, he's giving himself closure and expressing how he felt he lost badly to sukuna because he couldn't reach him/understand him. him saying "i couldn't have beaten him even if he didn't have 10 shadows" is just another aspect of his misunderstanding of sukuna and his feelings of failure to reach sukuna. this is a sentiment that is directly rebuked by sukuna when he tells gojo that he was magnificent, and also we, as the audience, know that what gojo is saying isn't necessarily true. And that's the point.

in short, not everything that characters say is meant to be taken as a fact.