Yes and no, it depends what you mean, but traditionally no
The concept of "magnum action" came into being with the Mauser "Commercial Magnum" action developed to house the .375 H&H and .416 Rigby cartridges created respectively in 1912 and 1911. This action was designed with a wider and longer magazine well. The magazine length was calculated to house the 3.75" .416 Rigby and was 3.84". This obviously allowed it to accept the shorter 3.6" .375 H&H.
Since then, a "magnum length" action has traditionally been understood as an action sized for the .416 Rigby, which typically means a magazine well length of 3.84".
The factory Winchester 70 "H&H Magnum" action has a magazine well 3.64" long. It works with the H&H length and profile cartridges - therefore it is a "H&H magnum" action, but it is too short for the Rigby cartridges - therefore it is not a magnum action in the traditional sense of "Mauser Commercial Magnum." It works well for the .375 H&H, barely but acceptably for the .416 Rem blown out shoulder, and is problematic with the .458 Lott straight case (which is why Winchester does not offer the .458 Lott as a factory chambering, despite the popularity of the Lott). It can be made to work well with the Lott but requires delicate re-profiling of the feed rails.
The feed ramp at the front of the Win 70 action can be machined away to lengthen the magazine well (just like the standard military K98 Mauser could be, resulting in the world famous .416 Rigby carried by Harry Selby), and the Winchester Custom shop did list for a while the .416 Rigby in their offering.
Visually, this is what the various Winchester 70 "long" actions look like viewed from the magazine well:
View attachment 373065
“Standard” a.k.a. “long” Win 70 action. Cartridge length up to 3.34”, narrow magazine box. Typical application: .30-06 family (.25/06, .270 Win, .280 Rem, .35 Whelen, etc.) and similar non-magnum cartridges.
View attachment 373066
“Short Magnum” Win 70 action. Cartridge length up to 3.34”, wide magazine box. Observe the thinner side walls. Typical application: Winchester’s circa 1950's original “short magnums” family (.264 Win, .300 Win, .338 Win, .458 Win, etc.) and similar belted cartridgeds (e.g. 7 mm Rem Mag).
View attachment 373067
"H&H Magnum" Win 70 action. Cartridge length up to 3.64”, wide magazine box. Observe the lengthened magazine well at front and rear. Typical application: H&H family (.300 H&H, .300 Wby, .375 H&H, 416 Rem, etc.)
View attachment 373068
Custom "Rigby length" magnum receiver, cartridge length up to 3.84”, wide magazine box. Observe the lengthened magazine well by machining away most of the feed ramp. Typical application: Rigby family (.416 Rigby, .45 Rigby, .416 Lapua, .378/.416/.406 Wby, etc.)