I'm curious, can "Strange Meeting" by Wilfred Owen be called a Heroic Couplet even if it doesn't contain end stopped rhymes?

I am afraid not. Heroic couplets are a pair of rhyming iambic pentameters. Thus the rhyming is something that is a basic constituent part of heroic couplets. "Strange Meeting" is therefore a poem that cannot be described as being written in heroic couplets, because the couplets do not rhyme.

You are right, however, to identify the way in which Owen does write this masterpiece in iambic pentameters. The problem is that, as we have already identified, there is no end rhyme and also the structure of these iambic pentameters allows for a more fluid division in terms of the thoughts that these lines contain. Consider, for example, the first three lines:

It seemed that out of battle I escaped
Down some profound dull tunnel, long since scooped
Through granites which titanic wars had groined.

Herer we have classic examples of iambic pentameters, but the lack of rhyme means that Owen is able to introduce his poem using three such lines rather than being constrained by having to make each pair of lines rhyme representing a single unit of thought. The poem's form thus allows Owen greater freedom in structurally presenting this diatribe against the horrors and futility of war.