Technically there is no Hi syllable in Japanese either. There is ひ, which phonologically is neither “Hi” nor “Fi”, but somewhere in between. The exact pronunciation varies depending on surrounding sounds, as well as the speaker’s regional accent.

So I wouldn’t say they really use WiHi. They write WiFi and they say “ワイハイ” which is the closest you can get to WiFi using Japanese sounds. It will kinda sound like WiHi to an English speaker.