I can qualify my statement since we had a guy who came from a TKD +Kickboxing background. He learned the kicks from TaeKwonDo but not patterns, and did both ITF and Kickboxing sparring.
Same level of athlete, same amount of years training.. Kicked my butt sparring. Guys who've trained far longer than him, equal athletic abilities but only TKD school I held my own against for 6 years.. Every single one. Suddenly I faced head movement, real boxing, and yet the same speedy kicks. It was not a pleasent experience. It felt like I was a car from the 80s facing a modern Tesla.
Your experience doesn't actually provide evidence for what you're stating. It suggests one of the below is true, but doesn't state which one.
1: The guy with the TKD & kickboxing background was an exceptional athlete/exceptionally skilled for his skill level.
2: Your school was not training you guys properly for an actual fight/sparring.
3: Your hypothesis-that TKD does not train you properly for an actual fight/sparring.
3B: your style of TKD (idk if you did ITF, WT, Norris-TKD or what) doesn't train you properly for an actual fight/sparring.
If you want to provide experiential proof for this, you would need a lot more than to extrapolate off your experience sparring that one guy. Couple steps that you'd have to do:
A: Have other people in your dojo spar him (I assume this happened but you didn't mention it). If you don't, then for whatever reason his style could just counter yours.
B: Spar with other kickboxers from whatever gym he learned kickboxing from. This will tell you if he was an exception for his school/more skilled than his level should be for sparring.
C: Spar with people from other TKD dojangs. Ideally from multiple styles, and not your 'sister dojangs'. This will tell you if your school is worse than TKD in general.
D: Spar with people from other kickboxing clubs/gyms. This will tell you if his school is better than the average kickboxing club.
For extra fun, there's also a 4th option of what your experience could mean.
4: TKD/your school is about as good as other schools, but kickboxing (or his specific school) is leagues above everything else in your area. To test this, you would need to add steps E and F.
E: Spar with people from other styles (karate/MT/Kenpo/WC).
F: Have people from other styles spar him.
Basically, it amounts to-you need to spar a huge number of people, both in and out of your style, to figure out if your statement has any actual validity.