Vegeta then proceeded to beat the shit out of Cell, and it's suggested SSJ Vegeta vs Initial Semi Cell would not be anything like that.
All the evidence surrounding SSJ Vegeta vs Semi Cell is quite vague, Android 16 only ever said that Vegeta has powered up considerably and Cell isn't fazed on him. The most we get is Trunks statement about Vegeta going to that state from the beginning.
In Dragon Ball, a 1.1 gap is a godstomp if maneouvered the proper way.
To me it seems like Cell doesn't know the full extension of his power when he absorbs 17 and 18. He mentions having to test his power against 16 and Trunks.
The most I can think of about it is the fact that he wanted to pressume his power with Android 16. He was aware that with his speed could avoid 18's self destruction and that he obtained that speed due to No. 17.
About Trunks, he was just refering to his current self (which he wasn't wrong as also Piccolo confirmed that Cell was weaker), and clearly, Buff Cell was stronger than even a FP Grade 3 Trunks.
Well Cell did get killed by Gohan in the end, so he's wrong as usual. He was only ever right about Piccolo, back when he was cautious about the people he fights, but after he absorbed 17 he seemed to have inherited his arrogance (Along with some of Freeza and Saiyan Saga Vegeta's as part of his DNA).
Realistically, Cell lost because the plot demanded him to lost, he was too haxxed for Gohan and almost his near-equal. He didn't even need to use strength to defeat him.
Was he also wrong with Goku?
Chapter: 398 (DBZ 204), P14.4-5
Context: after Goku fights Cell at full power for a bit
Cell: “Excellent, Son Goku! This is it! Battles aren’t interesting unless [the fighters’] true power is close to a certain extent like this.”
Goku: “Yeah…I think so too.”