Muay Thai and Taekwondo are both popular striking-based martial arts but they vary quite wildly in terms of effectiveness.
While there are definitely some good techniques in Taekwondo, such as the powerful spinning back kick, there are quite a few holes in the style which we see exposed in the following video.
For one, Taekwondo doesn't make use of the leg kick which is obviously a highly effective technique. We see the Muay Thai fighter land a couple solid ones in the video that clearly hurt his opponent.
Another deficit in Taekwondo is in punching techniques, both defensively and offensively. In Taekwondo competition, punching to the head is not allowed. This can create bad habits in Taekwondo practitioners as their extravagant kicking techniques may not be as practical if someone can simply blast you in the head with a straight right hand.
The only decent strike the Taekwondo black belt lands in this fight is the axe kick around the 55-second mark and it's really just a glancing blow.
Right after the ax kick lands though, we see the Muay Thai fighter land a great leg kick that the Taekwondo practitioner does not check, making the damage much worse.
Shortly thereafter, the Muay Thai fighter lands a straight left hand then follows it up with a big head kick which drops the Taekwondo guy and forces him to tap out. The head kick didn't even land 100% as the TKD guy was able to partially block it, yet it was such a powerful blow that it didn't matter.
This video is a good example of how these contrasting styles vary in terms of effectiveness. Compare the ax kick the TKD guy landed earlier to the final blow by the Muay Thai guy. Both were partially blocked yet the head kick from the Muay Thai guy still ended the fight.