Seems a lot more likely Shimano ended their partnership and decided not to supply them with the necessary components.
I got the impression from a conversation with a service tech from SRM USA when I had to get longer arms for the wife's SRM, the company machined the drive side arms in house and sourced the non-drive side from Shimano. Apparently supply issues have always been a consideration for the Shimano SRM's as the 170 mm arms supplied with her early production DA-9000 SRM were not actually 9000 series arms but were DA-7800/7900 series arms. Given the short supply/high cost of producing those arms, I would agree that it is more cost effective for SRM to proceed as you indicated, rather than continue with the custom one-off arms, not to mention it is nice to finally see the company update the electronics to reduce the footprint of the spider as well as finally offer an option recharge the batteries . Now if only they would offer self-service firmware update.
The latest series DA cranks SRM ever used were the 7800 cranks. These were used for the SRM DA powermeters since the 7800 series PM was produced by SRM in 2004. Initially SRM acquired 7800 cranks and did machine them themselves to mate with their powermeter spiders. They did this up until about 2013 when Shimano agreed to make and machine the cranks for SRM to use since so many of their sponsored teams were using them. These cranks were still 7800 cranks but were labelled as SR70 cranks by Shimano and had the striped anodized finish to match the 9000 DA series components that first hit the market in mid 2012.
With Shimano now making their own PM I would imagine that Shimano no longer wish to produce the cranks for SRM and have any chance that a team might prefer the SRM over Shimano’s PM. Hence SRM going back to using cranks of their own design and sourcing for Their Shimano style PM’s going forward.