The statement of "spring" appeared in the quarterly financial result presentation of Sega Sammy Holdings held on 11/06/2020. An English PDF summary of this presentation is available in English.
Yet the "spring" part was just a spoken comment, we need to check out Japanese language video presentation for this.
https://irpresentation.net/ir/6460/21Q2/index.html (probably you'll need a PC to watch this presentation)
The remark appeared in a part which is shown on page 35 of the English PDF. From 24:47 to 25:33, he talks about PSO2. And from 25:13, he mentions about NGS (which is not shown on the slides). Then follows the important point, at 25:28, he said "この春に予定して" (planned for this spring).
So at least the "2021 spring" comment is a fact. But as far as I know, no one found any other official comment from SEGA about "when in 2021". So maybe this Satomi COO's comment is something they want to forget about.
Well if those funding it want it in spring they saying we're getting it in that time frame isn't out of the blue. Japan normally isn't one for pushing their dates back and that is ESSPECIALLY true with Sega. I swear even when a game is clearly unfinished they still release it.