SiFive willingness to design with ARM hasn't changed, org has
SiFive's SOC design capability came from an acquisition of a company called Open-Silicon.
Open-Silicon was designing ARM cores before the acquisition, and as part of SiFive, continued to do so after the acquisition. So this announcement isn't really introducing a new capability, but might be an attempt to clear up confusion in the marketplace, so that prospective customers who want a custom ARM core don't assume that SiFive's custom SOC arm is a RISC-V-only shop.
It may also bring better focus inside the company to draw a clearer line between the SOC business and the RISC-V IP business, and give the two units distinct missions and goals.
A more intriguing possibility is that SiFive is making OpenFive more independent as preparation to spin it off. But I'd guess it's more about bringing more clarity to their offerings.