Keedounan said:
(and I doubt AT even came up with that concept at the time anyways)
That's a main reason I brought up Kibito as an example with how the evidence for him being around Gohan's level based on a single comment by Gohan compared to the wider knowledge we're given by the Z-Sword feat. In a similar manner, looking at things retroactively allows Zenkais to be fit into Part 1 material based on this comment from Kuririn:
Chapter: 268 (DBZ 74), P8.1-5
Context: Vegeta thinks Zarbon’s strength has fallen from the pre-emptive attack and brags about his own increase in power.
Vegeta: “Looks like my preemptive attack had an effect…Your stamina’s fallen quite a bit, hasn’t it? Zarbon, your death is close.”
Zarbon: “My d-death is close…!? Don’t make me laugh! My battle power is higher than yours!”
Vegeta: “Zarbon…Weren’t you supposed to know a lot about Saiyans? It wasn’t a good idea for you to carelessly treat me after I almost died…Don’t you think so? Saiyans can steadily increase their battle power each time they recover from near death…”
Kuririn: “So…so that’s it…! Now that he mentions it,
Goku also got stronger every time he had a fierce fight…Dam-dammit! So that’s why Vegeta’s ki has risen so darn much…!”
Which is further reinforced by this lining up with what Kuririn said when Goku fought Blue:
Chapter: Chapter 77, P.13.2-4
Context: Goku easily fights against Blue, not trying too hard. Blue admits he never faced such a threat.
Blue: “Gnnn... Oooh!!”
Goku: “Heh heh! No big dealio!”
Kuririn: “Goku’s amazing!
He's become even stronger than he was during the Tenkaichi Budoukai!!”
Blue: “You brat!! I've never encountered such insolence!! You’ll pay for this... Big time!!”
Goku: “So what? You want more?”
And the beating he got from Tambourine would certainly constitute a Zenkai, regardless of size.
As for the evidence as to why I think this, Roshi brings up his young self, let alone Mutaito, being helpless against Piccolo as a reason for why the current fighters have no chance, implying Prime Roshi would be above at least Tenshinhan if not he and Goku. Meanwhile, Daimao never expected to fight someone of Goku's strength in this era and cited him as doing about as well as a human could.
Chapter: 143, P6.3
Context: after Piccolo and Goku fight a bit, before Piccolo gets serious
Piccolo: “To think that there’d be someone in this world who could knock my back to the ground…I see…At that level, it’s no wonder my underlings couldn’t handle you…”
Chapter: 144, P7.1
Context: after seeming to kill Goku
Piccolo: “Naturally, this level is about as far as a human could go.”
Suggesting a chain of Mutaito ~ Goku (Post-Roast Fish) > Prime Roshi/Tsuru > 22nd TB Goku/Ten