Am I getting "stronger", or just more acid tolerant?

I’m a 48 yo male with about 3.5 years of structured training under my belt. Over the last year, I feel like my racing has improved, my FTP has gone up a bit (3.9 to 4.1, my time to exhaustion at threshold has improved, and my ability to recover from short over-threshold efforts has improved (surges in races used to kill me, now they don’t). Also the heart rate I can sustain for 20+ minutes has gone up (from 165 to 173) and my maximum recorded HR has bumped up from 180 to 183. But if I look at a plot of HR to power in, it looks exactly the same as last year. Am I correct in interpreting that this means that my legs aren’t really stronger, I’m just getting better at clearing acid and my cardiovascular system is able to sustain a threshold effort longer? I do spend more time doing threshold-type work than VO2 because it suits my riding style. But I have doing weights and some VO2 work during the season. Maybe I need to lift heavier, and/or do more sprints and VO2 work if I want my pedaling to get stronger? Am I correctly interpreting that unchanged HR to power profile as a loose proxy for unchanged leg strength?

Sounds to me you’re doing pretty well.

Without diving into specifics immediately. Does it matter whether you got stronger or not? All that matters is whether you got faster or not, right?

Also, what timeframes are you comparing when you look at your HR to power graph? If you, for example, look over a full year you could be missing nuances that are definitely there.