Why is it so controversial when someone says “All Lives Matter”? (2016)

The argument is that what is really meant when "[insert particular group] lives matter" is stated, is something like "we all agree [insert groups in general] lives matter, but [insert particular group] lives don't seem to matter as much (i.e. the general group moral axiom is not held up for this particular group)."

That gloss is not the same exact gloss that the Reddit commenter makes, however, this has to be what was meant. How else to interpret the point about an implied 'too'?

The primary problem with the argument is pretty simple (as the commenter makes it clear what their assumptions are). Namely, a non-trivial number of people don't believe that any particular group is being snubbed in the way that a speaker of the gloss above would assert. Since there is a pretty solid correlation between people who believe that and those who re-assert the general group moral axiom, such speakers are likely doing something more than simply ignoring or passing over the implied too from speakers of the gloss above. I won't wade into all the possible meanings/uses that speakers might have for any of the above types of statements. The range is very wide. Some are likely making the relevant statement because they want to signal which group they are with, etc etc.

The Reddit commenter clears up a good a bit of the confusion for people who already agree that a particular group is being snubbed. Perhaps that was what the person who asked the question wanted resolved. If they wanted it resolved tout court, though, then the argument presented isn't satisfactory.

In reality "Black lives Mater" and "All Lives Matter" are just abstractions for complex ideas that can change over time and location.

Talking about "too"'s is a good example of the childish level that Reddit sits on.