Actually Syd's closest 400h time to her 50.07 PR was 53.60. She ran 22.39 and 50.07 as her 200/400 outdoor season openers then opened up in 53.60 over the hurdles about a month later. Her outdoor PR's actually score worse than her indoor PR's because she never raced more than once outdoors. If she had run the 400 even once or twice more, she would've definitely beat or come very close to the original WJR of 49.4 (based on her HS progression, indoor 400 WJR of 50.3, and 400h improvement that season). Same with the 200.
Separately, Syd's relay splits have always converted very well to her flat times. In fact, she's split a ton of 4x4s slower than or exactly equal to her actual 400 capability too. I'm quite confident her 47 split last year will result in no slower than 48 mid speed. Besides, both Bobby and Syd have stated that she was capable of 49.X over 400H last year so that is further indication she was at 47 high/48 low 400 in 2022. And you can bet they wouldn't be talking about the 400 WR now unless it absolutely wasn't in reach.
People look at Syd and think she's a hurdler, forgetting that she absolutely is a prodigy over the flats too. She has only ever seriously raced the 400 indoors...and holds every indoor 400 record she could (WYR, WJR, HSR). There's never going to be definitive proof that she can break the WR until she actually does it but based purely off of past records and progression, we have never seen a greater 400 talent than Syd (and Salwa Eid Naser). Given how young they both are, you can bet they have faster times in them than what we've seen till date.