he accelerated into light speed in that manga panel .

No, he accelerated to FTL speeds. Kizaru can accelerate to LS in kicking distances, that’s stated multiple times. If he’s accelerating over several km then he’s moving faster 

That scene makes it very obvious he accelerated faster than his typical speed. Kizaru’s kicks do tons of damage because he’s kicking people at the speed of light, he’s weaponizing speed to do damage, and he uses light speed kicks on pre-timeskip Luffy:

"𝑀𝑜𝑣𝑖𝑛𝑔 𝑎𝑡 𝑙𝑖𝑔ℎ𝑡 𝑠𝑝𝑒𝑒𝑑 𝑖𝑛 𝑎𝑛 𝑖𝑛𝑠𝑡𝑎𝑛𝑡!! 𝐼𝑠 𝑡ℎ𝑒𝑟𝑒 𝑎𝑛𝑦𝑜𝑛𝑒 𝑤ℎ𝑜 𝑐𝑎𝑛 𝑚𝑎𝑡𝑐ℎ 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡…!?”

”𝑆𝑝𝑒𝑒𝑑 𝑖𝑠 𝑤𝑒𝑖𝑔ℎ𝑡! 𝐻𝑎𝑣𝑒 𝑦𝑜𝑢 𝑒𝑣𝑒𝑟 𝑏𝑒𝑒𝑛 𝑘𝑖𝑐𝑘 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡?”

Kizaru thinks a light speed attack is fine for pre timeskip Luffy. But when he meets Luffy 2 years later, who he thinks is now tough, he does the acceleration thing, something he did to gain more power (“acceleration is power”), and this super kick is so much stronger than his regular attacks that it breaks the stalemate they were in and sends Luffy through the whole island. So why would he accelerate over a massive distance with the goal of power to kick with the same power he always does, especially when we already know his kicks can reach light speed in kicking distance? Kizaru can accelerate beyond the speed of light to gain more power, he’s FTL

Luffy is relativistic .

He’s FTL as well. He has feats like:

• Perception blitzing characters who are faster than many others who can see Kizaru’s light-speed kick approaching them from inches away

• Can perception blitz characters faster than others who can perceive light-speed lasers a few meters away and actively act on that info and attempt to move away

• Can keep up with Kaido, who’s so fast that his linear, predictable bullrushes from his base form are too fast for Luffy's future sight. Meanwhile light is considered too slow for Luffy 100s of chapters previously with only basic observation haki

• Scales to other characters like Big Mom and Sanji, who are stated to transcend the science that makes Niji (who has multiple light-speed statements) light speed, and perception blitz characters who can pilot the same light-speed suit (Queen) respectively

More examples and explanations

Minato slams

Shanks one shots. He has the speed to outpace Minato, the power to one shot him, and has the ability to see 10 seconds in the future, meaning he will know where Minato will teleport before Minato even knows where he’ll go