Seems to be one of those grey areas.
I think Super Saiyan Gotenks is definitely Super Saiyan 3 tier regardless. Maybe there just wasn't an explosion of power like it was with Super Saiyan 3 Goku. Fusing into Super Saiyan looked smooth, whereas transforming into a Super Saiyan 3 looked intense for both Goku and Gotenks.
Besides that, nearly everyone sensed from a tremendous distance seemed to unleash a huge amount of power before being sensed.
Super Saiyan 3 Goku:
Super Boo:
We see Goku and everyone on the Kaioshin realm's reaction to Super Boo's Chi in the following chapter
after that explosion of power. Piccolo shit himself immediately after.
Gohan:
Self-explanatory.
Piccolo sensed Kid Boo's Chi disappear. There was no visible power-up, but he seemed to be exerting himself quite hard to stop the Genki-Dama. Outside of sensing Super Saiyan 3 Goku, Gohan doesn't seem too adept at sensing from a distance. He was unaware that Gotenks' Fusion returned to normal, and Piccolo had to inform him and the kids that Kid Boo's Chi had disappeared. The times when Super Saiyan 3 Gotenks had an explosion of power was when he transformed and when he ripped a hole in the dimensions.
The Chi being huge is relevant, but I think the emphasis lies in how that Chi is unleashed.
Totally something I thought of on the whim literally 20 mins ago, lol. It's a reasonable enough explanation for me, though.