~amd64 keyword on a single package

Greetings,

This is the wifi interface on this new system, as shown by lspci:

04:00.0 Network controller: Intel Corporation Wi-Fi 6E(802.11ax) AX210/AX1675* 
2x2 [Typhoon Peak] (rev 1a)

The current stable kernel seems to have no support for this device, so I'd 
like to know whether a later version will include it. Therefore, after reading 
the wiki[1], I set package.env like this:

# cat /etc/portage/env/testing
sys-kernel/gentoo-sources  ACCEPT_KEYWORDS="${ACCEPT_KEYWORDS} 
~amd64"    [2]
# cat /etc/portage/package.env
sys-kernel/gentoo-sources testing

Emerging the kernel sources then produces this error:
!!! Problem in 'sys-kernel/gentoo-sources' dependencies.
!!! "/etc/portage/env/testing", line 1: Invalid token 'ACCEPT_KEYWORDS' (not 
'=') portage.exception

What have I misunderstood? And is there a better way?

1.  https://wiki.gentoo.org/wiki//etc/portage/package.env
2.  I also tried it without the ${ACCEPT_KEYWORDS} part, with the same result.

-- 
Regards,
Peter.