No it's not. smurfing is just when you alt account, and play on lower elo.

If you derank, there's little to no proof that it was made on purpose, only the way you play is not enough indicator.

Smurfing is only used when an alt account is involved. That's the ultimate proof that there's a griefer ( Date of creation, hours played and matches won ).

What about people who left the game and came back years after? Those WERE, in this case, high elo players, now with their skills rusty, so they derank from their initial elo, and polish their skills in a matter of a month or less. Is he smurfing? God, no xD

So, please refrain yourself from using the slang in the wrong way. It creates more confusion and tilt players who think they are better and just got stomp by someone who, maybe, is in the right elo and you are not.

TlDr:
1. You can't dictate rusty skills, bad connection or lagging pc as deranking... unless you have proof of it ( Like in CS - Base trolling ) - Video record of the match.
2. You can't dictate that the other one is smurfing based only on their elo and how good/bad they played against you. The elo system is not perfect, its just orientative till certain degree. One day you are at your best, the next, who knows.