Each polish division and most brigades had 4 40mm guns (some 8) and probably a number of aamg's (not sure but it seems the arty's lmg's might have served as aamg's).
http://www.orbat.com/site/ww2/drleo/...ons/_1939.html
Furthermore, there were close to 50 independent aamg companies serving with the different polish armies.
http://www.orbat.com/site/ww2/drleo/...1939/_army.htm
These armies also fielded a number of independent 40mm platoons and some companies of heavier calibre AA guns. These numbers do NOT include the AA assets for 'fortified zones' nor those of the AA brigade of the anti aircraft command nor those of the coastal commands
http://www.orbat.com/site/ww2/drleo/.../_army_aa.html
So there was plenty of flak available to the poles.
With up to 8 aamg's per platoon (in others 4, still looking for the exact number in the independent cie's) in each aamg cie and since 2900 points buys roughly a regiment of polish infantry with support the amount of AA you saw is not excessive.
Narwan