That Defence Connect article is from Nov. 2019, so a bit dated from a programme perspective, not to mention that member of the US Executive branch the article referred to was the then-Sec of Commerce, Wilbur Ross. Aside from an inclination on my part to treat anything coming from him as suspect until proven otherwise (he apparently had a number of conflicts of interest and/or non-disclosures when he was Sec during the prior administration), the Secretary of Commerce is no where near as important as the SecDef or SecState, especially for something like a FMS transaction of strategic hardware which will be part of the US nuclear triad.
Relating to that, as mentioned the B-21 Raider is to have a role in US nuclear deterrence, as a replacement for the B-2. Also, such an arms sale would need approval from the US State Department, and one of the considerations they would have is what would be the impact of such a sale, asking questions like, "would this kickstart a regional arms race?" or, "would this accelerate hostilities?". Even if approval from the State Dept was granted (which IMO would be unlikely for a strategic asset like a LO intercontinental nuclear/conventional strategic bomber) the sale would then need to get Congressional approval. I could easily see members of Congress working to pass something similar to the Obey Amendment if significant noise/interest in purchasing the B-21 gets exhibited outside the US. Now if there were current, reputable US sources which indicated that the current administration and minority party were interested in FMS of the B-21 Raider, the situation would be different. So far though, it seems that the interest in such a sale, as well as the potential for it to occur, are all coming from Australian sources. Further, it seems that these sources are not giving what IMO would be due consideration to objections various elements of the US would have, since (at present) no US laws currently exist which prohibit such a sale.
I am not going to re-hash the significant cost issues which others have already touched upon if such an order were to be attempted.
However, I do feel the need to point out something which AFAIK no one else has commented on. IF the RAAF were to place such an order, and IF it was to get all the way through the requisite FMS and other restrictions, when would be the first delivery, IOC and then FOC? Under current plans, the B-21 Raider is expected to enter service in the USAF in 2026 or 2027, with between 80 and 100 aircraft ordered. Depending on what the full scale production rate is, it could take years before the USAF order is completed. This would be dragged out even more if the US does end up increasing the total fleet size up to 200... I would therefore not expect any B-21 Raider deliveries to the RAAF to be able to occur until 2032 or later, again assuming that a sale was placed and approved.