Captain Cadaver said:
Boo was there at the time, whilst Piccolo wasn't. He also picked Gohan before Piccolo, despite the former only showing signs of superiority in the filler scenes you're disregarding, so that says little.
If it was just a matter of Boo being there Goku would have also picked Goten and Trunks, who he was considering until he went like "nah those kids aren't ready yet" Seeing as in his base form alone he was giving Piccolo a good spare, Super Saiyan Gohan probably is considerably above Piccolo.
I just don't see what the big deal is in retconning the Base Saiyans back to Boo Saga levels, theres tons of evidence pointing to them being at that level while the only evidence of them having super powerful base forms is in Toei material that is almost certainly not in Toriyama's script. Goku absorbing Super Saiyan God was retconned. No way is all the U6 fighters god tier, no way is Good Boo such a relevant and powerful ally if this is the case, no way Piccolo who is all but stated to be weaker than Good Boo contend with someone well above Base Saiyans. no way does a slacker like Gohan go from getting holes put through him by First Form Freeza to fighting on equal terms with Goku . We accept how often Beerus power gets retconned so why can't the same be said for Goku/Vegeta who were apparently suppose to be well on there way to surpassing Beerus in the ROF movie until Toriyama decided he wanted to continue the series.