The blog post is unclear. On a technical level (on the preload list), is the enforcement at the TLD level or is it just a legal requirement to submit all .gov domain names to the preload list? If the latter, any plan to move to the former?
Not quite either one -- it's technical enforcement by the TLD, but still done on a per-domain basis (this doesn't affect state/local .gov domains, or legislative/judicial .gov domains). The dotgov.gov program will forcibly preload new domains, but it's not feasible to just submit ".gov" to the preload list right now.
The DoD has a massive PKI system already and makes the assumption users of its sites have the appropriate CAs installed. (Home access typically requires installing a set of them, typically bundled separately or in an installer)
DoD uses https and crypto at the transport layer in SIPR. Lots of "type 1" crypto as well, which is its own special thing with NSA issued hardware crypto keys.
This is a GSA initiative, not an 18F initiative. But 18F has a recent post detailing executive branch progress on HTTPS that may also be relevant:
https://18f.gsa.gov/2017/01/04/tracking-the-us-governments-p...