You’re indeed correct. The first English Pokémon TCG product was the Pokémon Demo Game Pack, which was given to some MTG distributers in December 1998 (and also given at some events later on in 1999). These Demo Game Packs contained 24 Shadowless cards.
The Trainer Deck A and Trainer Deck B were distributed shortly after (unlimited design with red-bordered back). And only then the English 1st edition Base Set set was released on January 9th, 1999.
So although Shadowless; unlimited Trainer Deck A/B; and 1st edition Base Set cards were all released almost simultenously in those two months of December 1998 and January 1999, the 24 Shadowless cards of the Pokémon Demo Game Pack were the very first official English TCG release.
(Of course there is also the Protostoise (a prototype Blastoise) from Summer and Fall of 1998, but it was never released nor intended for release, so not too relevant here.)
Greetz,
Quuador