score:3
I tried it in my recent trip to Seoul. You can buy the SIM card from KT olleh and use it for tethering. I asked the staff at counter if I can use tethering, and she kindly replied "Yes".
And now I'm writing this answer on my MacBook via tethering. It's 4G/LTE, unlimited, and likely no speed cap or restriction, but for me it seems a bit slow, but OK.
By the way, the linked page says the following but it is NOT correct:
SIM card rental
They rent out a data-only prepaid SIM and a general prepaid SIM with voice and data. Both SIMs include "unrestricted" data use during the rental period. You need to link your credit card and in case of loss ₩ 10,000 will be charged. Remember, you need to give or sent back their SIM. Rental rates for both SIM cards are:
for 5 days: ₩ 25,000
for 10 days: ₩ 35,000
for 30 days: ₩ 65,000
Actually, only prepaid SIM with voice and data is a rental type. Data-only SIM is purchasable and doesn't need to be returned. I purchased the data-only SIM. The price is the same for both cards, for your information.